I've talked about this in the past, but it needs to be said again.
I am contacted regularly for possible work for hire on here. I can tell you 95% of the time, it is people asking for art for no compensation.
1. No compensation or "Exposure", as it's regularly referred to as, is a joke and an insult to any artist. If you want work, pay for it. I cannot tell you how many times I've been contacted for Free work. It's a daily occurrence, sometimes several times per day. NEVER ask for someone to do it for free. If the artist is good enough to ask to do work for you, they're good enough to be paid. And as for the "Exposure", if they're good enough to ask to do the work, it's likely they have a following and don't need YOUR exposure.
2. Percentage is a joke and an insult just as much as exposure. There's NO incentive for someone to work for a percentage of the sales because unless it's a proven property, there's no guarantee it's going to make any money. You know when this IS in your favor? When you're doing work for major company and a major property and even in that case, they're likely to just pay you "work for hire" and keep the profits.
3. Try and pay within the artists rate. Now this one can be worked out. I am willing to work within someone's budget unless it's far too much work for what I'm being paid. The plus side is you're willing to pay and honestly, that's great. If I can't do it because the amount doesn't work for me, there's going to be another talented person behind me who can. You're definitely in a good category in my eyes just being willing to pay. I'm sure if you could pay me the rate I requested, you would.
Let me explain something to you, When I do a Kickstarter for my projects, there is a large portion that strictly is reserved to pay my Colorist, Letterer, the guy formatting the book for digital and anyone else involved in the process. And it's what they ask for. I don't try to haggle down. If tomorrow my Colorist said "On the next book, I need to raise my page rate to $XXX per page", I would prepare for it and get it together.
PAY the artists. Don't ask for Free work.