A quick web search seems to suggest that this isn’t a great idea. While saran wrap has been tested effective in stopping viruses (http://www.ninecircles.ca/Infoline/FAQ/2_How%20can%20I%20protect%20myself%20against%20HIV%20and%20STD.htm) it seems that there are other problems associated with using it as a condom. It’s rather hard to apply properly, can cause extreme irritation, and breaks easily. Something to consider; If you can’t afford to buy a proper condom, can you afford the increased possibility of becoming a parent?
http://www.wackyuses.com/wf_saran.html
http://www.canadiancondom.com/LearnAboutNF/BarrierMaleCondom.htm
http://www.scarleteen.com/boyfriend/adv_wrap.html
http://www.chebucto.ns.ca/Health/TeenHealth/HSex/bcmiscon.html