hmm seems to me that your not looking to start a contest but looking for someone that you want to hire that edits in a style you like. A contest video is usually done by a creator who is inspired to do a song to an anime, like anneke said. If 20 people sign up for your contest and only one of them wins those other 19 people did a lot of work for little reward. Yes, they could put the work up but that would be 20 vids up for download all with the same anime done to the same song. That wouldn't fit well with a lotta people who complain about Linkin Ball Z vids.
If you want to make this into a contest, just ask everyone to make a 10 - 20 sec short of a segment of the song that you'd most want to see done. Then hire the winner of the contest to do that song. Whether for money, or if they are actually inspired by the song, your getting your vid for $100 the way you want from the creator of your choice.
I know this is frowned on by many here, but me, I wouldn't mind doing this for $100 if it help me run my website (I have a limited transfer rate, and more gb worth of transfer means mola). Actually I may mind, since I'm in the middle of two projects and may be starting a third. Partially because I'm not inspired enough to do it, and the other half being that I just finished an Eva video.
Well those are my thoughts on this, any others?