Cheap version : Buy an inverter plus standard PC psu. Then deal with the hassle it gets you (potential audio noise, size, low efficiency).
I recommend you do it right the first time and stump up the cash. I read someone's .sig somewhere that said something along the lines of "There's never the time to do it right, but there's always time to do it over." How true. Since time is money I reckon you can replace the word time with money in that saying and it'll still hold true.