The video itself would not be earning money, so obviously it wouldn't be a percentage deal, and obviously I also can't afford to pay much.
The specific issues that come up...
1) Classical music: how does this work? Holst's _The Planets_ as a piece of music is clearly public domain, but I would presume that any given performance of it (Stokowski's , John Williams, the Polish National Orchestra's, whatever) is not, and thus I can't use it without permission as background for a video that I intend to use as advertisement (whether the video is free or not). Who would I contact? How much would something like this cost?
2) Japanese anime/videogame soundtracks. There's some really good music in these, and in some ways I've noticed the Japanese having more sane policies towards IP -- and more restrictive in others. Here, of course, I have no idea WHO I would contact, or how. (the games I'm particularly interested in at this point are Mana Khemia, Wild Arms 4, and Dirge of Cerberus)