You can get a solid used Shure mic for around $30-50. That should easily be enough for a DJ. Check out some of the other posts on here, Rio. Shure, Sennheiser, AKG, and Audio-Technica have been getting some recommendations. AKG and Audio-Technica may be out of your price range though...
You my also want to compare the Shure mics with some offerings from Nady. They put out some decent equipment. He's a DJ, so he really just needs something clean. He's not singing into it or anything, but it's still a good idea to make a solid investment. Especially since you shouldn't run anything over $100 at most, even if he buys it new.