I bought a couple of ASICMiner block erupters off eBay for about $40 each. They do 335kH each.
Decent enough to get you started in pool mining on any SHA-256 currency, I think BitCoin has just too high a barrier to get into right now but that's just my 2 Lincolns

Plus if you're new to all this (like me

) It's best to keep your investment low until you know what you're doing with a command line and compiling code.
I think even to pool mine BTC you would need a minimum of 5gH just to even make it psychologically viable. But I'm a newb at this and it's just my Google-ated guess.
