If you've already got a frame, building might be cheaper assuming you can get good used components. Sometimes components will cost almost as much as a bike. On thing to note, older frames sometimes can be a pain to take modern stuff and sometimes might have higher gearing than your friends mountain bike.
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Bikes: 1996 Eddy Merckx Titanium EX, 1989/90 Colnago Super(issimo?) Piu(?), 1990 Concorde Aquila(hit by car while riding), others in build queue "when I get the time"