i don't know if the words cheap and plasma cutter can be used on two different websites.....let alone in the same thread! especially if you want one to cut 1/2" with.....
try ebay - search for: plasma cutter - I bought one of the cheap chinese units a few years ago... works fine, cuts 3/8 no problem (slow, but no problem) cuts 1/2, but its not pretty...
just realize, you're taking a gamble... if it breaks, you're probably on your own.....
You can get the Chincom "stuff" on ebay. Most of what ive read on them hasnt been the most flattering, but some have been happy with their purchase. Have a look on Weldingweb and see whats there.
Is there a reason you want plasma? Cheap and plasma dont go together as the others have said. You'll also need good air filtration or youll go thru consumables quickly.
A Oxy/acet torch with bottles can be bought for under $500. ~$200 for a brand name torch (Victor) and bottles for under $300. A torch will also give you a more flexable tool than a plasma. You can weld, braze, cut (thicker) and probably most important heat with a torch. Just something to think about.
I'am fabricator that does a fair amount of stainless steel work etc and we use the plasma quite a bit.You should check the price of consumables (tips etc) when you buy a unit.If you use it a fair amount they really add up.Mind you we cut up to 1 1/2" stainless with them.Even light stuff will burn up parts not a big deal but some machines have way cheaper parts.
Just like Scooby074 said cutting torch is by far the cheapest way to go. Although the Acetelen is somewhat high. I have used Oxy/Propane for several years and love it. It is by far the very cheapest and still with the right cutting tip, I have cut up to 1" 1/2 metal. Takes a bit longer to heat up then Acetelen, but only about 20seconds longer. No big deal. And I use the propane cylinder from my BBQ grill. Good luck