Ok Ive been dealing with cavaliers for about 12 years now and have built some nice ones too, there is absolutly no way your going to boost a cav for 300 period. go to Jbody.org that is a cavalier/sunfire community.
there are many other things you'll have to do than just throwing one in there you have fuel issues, computor issues drive line reliability, and motor conditions to worry about.
The cheapest I've found kits for was 1500 and that was just the basics turbo, manifold and oil lines and thats not including the BOV intercooler FPR injectors or intercooler lines, that kit is 3500.
Tell your friend to get a header exhaust and intake and learn about cars before he goes and buys a turbo kit and kills someone