Typical Paid Percentage Per Design?

Okay artist designs a drawing, partners with a digitizer who digitizes it and sells the design on products or outright. I understand that the designer makes a percentage of sale each time the digitized designs sells.

My question is, what is considered a fair percent to pay the artist per design? Does this vary due to complexity? Any information you can provide on the payment between artist and digitizer would be great. Thank you.