a buddy of mine simply removed the derailleur, shortened the chain, and used one of the gears on the cassette. he also took the outer ring off the crankset, and moved the inner ring to the outside of the crankarms. he liked the gearing (42/15 i think?), and the chain tension and alignment was right on (close enough for s.s.). sure, he still had all the other gears on the back, but it was his beater, ran good, and it cost him zero.
if you need some chain tension adjustment, get a 1/2 link like stated above. switching the front ring in to out also gives you some adjustment with chainline.
you said you wanted cheap. for free, or the buck for a link, it's worth a shot...