$10/month for VDJ Grooveshark for stuff I'll only ever play once - better than buying crappy special requests for a wedding, which will never see the light of day again after that wedding. Also good for walk-up special requests at the event - Great-Grandma Ethel Sue Ellen's first waltz, etc. As long as you have internet access at the event, you have access to "15 million" songs, or whatever it is Grooveshark claims. I think you must have a legal version of VDJ though, although they may sell subscriptions without VDJ... Not positive.
~350/year (IIRC) for annual subscription to ZipDJ.com - GREAT selection of music from many genres (dance, club, Top 40, country, adult contemporary, Latin, house, trance, rock, and more). Unlimited downloads. The popular dance songs usually have remixes available, and they have some DJ tools (like instrumentals and acapellas, so you can make your own mashups).
A few dollars here and there buying compilations.
A few more dollars buying old CD's or vinyl at garage sales or Goodwill.
So, less than $1000/year, but your mileage may vary.