I'm not sure how much you provide beyond art, also an sdk? It sounds like you are essentially a publisher. Since you are giving a royalty advance in the form of art, software support, handling distribution and possibly marketing. You are then giving devs a royalty rate of 70%.

My understanding is that publishers usually take more than 50% of revenue. I don't know if you should take that much or restructure your payment system, and honestly I don't think being a publisher for tiny xblig like games makes sense.