Cheap Singlespeeds. SS $100 bike, $200 bike. Walmart thruster. 6KU fixie. Bikesdirect Dawes Streetfighter SST

Singlespeed bikes.  The simplest type of bike.  If you live somewhere flat, hate maintenance, and like to pedal hard on hills, these are for you.

DON'T get a $129 walmart Thruster!!!

It's a pile.

The cheapest, decent quality singlespeed  is the $200 6KU.

The frame and fork are high tensile steel, which is a bit heavy, but is quite durable.  The wheels are nice, with super strong deep V rims, and good hubs.  The fork is threadless.  It fits fatter tires and fenders.  The cranks are pretty nice.  This is a good bike for $200.

There are even are a ton of frame and wheel color combinations to choose from, too.

http://www.citygrounds.com/fixed-gear-single-speed-bikes#brand=6ku

The main crappy things on this bike, things to upgrade to make it a more durable commuter.  Are the freewheel, it has junky chinese one that will fail.  The freewheel is the gear on the back wheel, that coasts and makes the clickity noise.  Good quality  ACS, Shimano, or GT ones are $15-25 dollars.

And the tires.  Good quality, flat resistant tires are key to a bike you ride everyday.  Continental gatorskins, Schwalbe Marathons, and Panaracer Paselas are all good tires, between $30-40.

At $230, you get the Bikesdirect Dawes Streetfighter.

This bike has a cromoly steel frame and fork, which is lighter and stronger, better quality.  Realistically, most people won't bend the hi-ten 6ku frame, and it is maybe 2 pound difference.  But the streetfighter is definitely the better frame. 

The tires and freewheel still need urgent replacement.  The seat, seatpost, bars are a bit junkier than the 6ku.  Not a huge deal, as these are parts many bike riders will replace anyways to make their bike fit better and more comfortably.

The $230 Dawes SST is similar bike, except with drop bars

Bikesdirect Motobecane Mirage SL $450 STI 700c road bike

Bikedirect bikes.  Loved and hated.

The Mirage SL is the cheapest road bike I would consider from bikesdirect. If you live in an area that has a good used bike marketplace, you can likely find a better quality, barely used bike.  Good brands to look for are trek, giant, specialized, trek.  Search craiglist from $250-500.  Post up what you find on https://www.reddit.com/r/whichbike/ and bike nerds will give you advice,

If you like somewhere with a poor used bike market, the $450 mirage SL might be your best option for a road bike under $500.
http://www.bikesdirect.com/products/motobecane/road_bikes/mirage_sl.htm

This is a bike shop quality bike.  From the aluminum frame, to carbon fork, to beefy wheels with cassette hubs. Shimano Claris STI shifters with 2X8 , to FSA compact 50X34 cranks, this is a current, decent quality road bike.  It compares favorably to bike shop bikes costing $700-800.  For example the $770 Trek 1.1

Sure, the tires, bars, and seatpost are a bit nicer on the Trek, but those are things you might change anyways.

One big advantage of the bike shop bike, is that it is assembled and adjusted by someone who knows how to work on bikes, and uses proper lubes.  This makes a huge difference!  If you bring your BD mirage SL to a bike shop, to have them put it together and adjust it, expect to pay $50-75 labor, and it's worth it!  If you are mechanically inclined, and willing to invest $25 or so in tools and lube, and willing to do a little research, you can do it yourself.  Mostly putting on the front wheels, pedals, handlebars, seat as far as assembly goes, pretty easy.  Adjusting the gears, and brakes, and truing the wheels is a bit trickier.  You will need: good bike chain lube, like triflow or prolink, grease, car bearing grease or white lithium is fine.  Metric allen keys.  Cable cutters.  Eventually, you'd want a chain tool.

Unlike other budget options, like department store bikes, this bike comes in many size options, so you can get a bike that fits you if you are tall or small.

The main cruddy thing for a commuter on the motobecane, are the poor tires.  They get flats easily.  Two of the more popular, fast, flat resistant commuter road bike tires are the continental gatorskins, and the Panaracer Paselas.  Gatorskins start at $40, Paselas for $30 for the PT puncture resistant ones.
This bike can fit up to 28c wide tires, 23c would be super skinny, 25c in the middle.  Fatter tires will be more comfy, and less prone to flats, if slightly slower.  Most commuters run 28c or fatter.

Other wise, it is a good bike, and should wake up call to people that try to sell 90's STI roadbikes that aren't particularly fancy or special for more than $400.

One thing about this bike, and road race style bikes in general, is that is has low profile brakes, that make it tougher to fit fenders, or mud guards.  And make it so you have to run skinnier tires, 28c max.  And 25c if you want to run fenders.   Hybrid, touring, or cyclocross bikes let run have fatter tires and fenders.

This bike is good for someone that wants to get into roadriding, or needs a very fast, efficient commuter for long distances, and is willing to give up some comfort.  The best upgrade you can do to this bike is new tires.

26" Schwinn Sidewinder $149 26" mtb, shimano 3X7 gears.

This is the best bike you can get at walmart for $150, if you are a medium sized person.  Every magna, roadmaster, mgx, huffy, next bike that is cheaper than this is a total piece of garbage.  This bike is actually serviceable, functional, and worth fixing.

The Schwinn Sidewinder is a 26" wheel mtb, with 3X7 shimano parts for 21 gears.

It's parts are pretty decent:

Shimano tourney 3X7, derailleurs and grip shift shifters.

Alloy linear pull brakes and levers.

Wheels with alloy double wall rims and alloy hubs.

It compares pretty favorably to a Trek 820, $380 bike shop quality bike.

http://www.trekbikes.com/us/en_US/bikes/mountain/sport/820/820/p/1008600-2016

The gears, wheels, brakes are the same.  The trek does have better tires, thread-less fork, nicer bars, seat, post.  And it comes in many sizes.

This gets to the main thorn with the walmart bike.  It only comes in one size, 17", or medium, good for people 5'7"-5'10" about. If you are shorter or taller, you are out of luck

The bike shop bike has 5 sizes, XS, S, M, L, XL.

Also, the bike shop bike is put together by someone that knows how to assemble and adjust a bike.  This makes a huge difference!  If you took the Walmart schwinn to a bike shop to have them assemble/tune the bike so it works right, labor would likely be $50-75, and worth it.  That said, bicycles are not rocket ships, and someone with good mechanical aptitude, and willingness to do some research and buy $25-50 of tools and lube can do a good job.

The main crappier parts about the Schwinn is the threaded fork and stem.  And crappy suspension fork.

The handlebars and seatpost are also heavy, cheap steel, where the trek are aluminum.  

Neither of these are that noticible to basic commuter bike rider.

The tires are a weakness.  The stock tires get flat easily, and wear out fast.  Replacing them with flat resistant tires would be a huge upgrade for someone using the bike to get to work.  The knobby tires roll slow on the road.

$13 Panaracer 2" touring tires are a good choice

http://www.niagaracycle.com/categories/tires-pan-tour-26x2-0-wire-bk-bk

Otherwise, just use the bike til stuff breaks.  Most likely things you are likely to run into issues with are the chain. $7 KMC Z50 chain http://www.niagaracycle.com/categories/kmc-z50-chain-6-7-8-speed-1-2-x-3-32-x-116l-silver-brown

Seatpost.  Check the stamp at bottom of post, likely 27.2mm, and get alloy Kalloy seatpost, around $20.  

For people that are not medium sized, or people that live in areas that have good used bike markets, a used 90's MTB from Trek, Giant, Specialized, Cannondale, etc, for less than $150, is a better deal.