What she said!
Percentage thing won't work first because it never happens. I had something lik 10 contracts like that, you know how much $ I got from that sales % shit? Zero! Happens to others too, so they don't really believe in that shit anymore.
A lot of factors are in there: is it cd or digital, how and how much you pay, how much that artist spend on that track... if it's months for sure he will ask for good money. Some of them love that track so much so they won't give it just like that. Would they like your idea of compilation itself? Some of them are living from music so they don't care for free music.
Some of them will give you for free and ask for few copies, some will ask money and 10-20 copies, some will ask to fix them booking in your country for such amount of money if they give you track. Some of them don't like selling rights because at some labels it get's re-released over and over on 'VA - Top of Bullshit Trance' compilations without even being asked or announced.
Some of them will simply reject you because you are new/unknown for them and they don't trust you or don't trust you can promote their music enough. I don't want to scare you, just be prepared that your dream artist wont give you or simply dont have any unsigned tracks for you. From personal experience I reject digital compilations always, I prefer or CD or free download. As for digital EPs have more impact and promotion for artist.
First thing first is set up your budget and know how much money you have for pressing, mastering, cover art, sponsoring, shippings and taxes to online shops and artists. It will probably take couple of years to get that money back/be at 0.
Back in the days when I ran UAF Records everything was for free download, everyone gave music for free and even did coverarts and mastering for free, ofcourse not so professional but label served for promoting new artists and it really did that.
Let's say you do free download compilation and you get tracks for free you will still need around 500 euro for professional mastering and cover art design. Make good story and approach to your dream team and you will see where it takes you. Good luck