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.
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