So I made a program for this guy for an upfront amount of money. This program is now in the top 10 most popular list in its category on download.com and is in the number one spot in its subcategory.

Recently, I've made a much more advanced version of the same program and am currently in talks with the same guy to sell the new program, but this time I want a percentage of the profits. He's offered me 20% of gross income (20% of about $5-10k/week) that is generated with the new program. His company/website is sort of known as the place to go for this type of software and he has a large customer base. While I'll make more with this payment scheme than I did with the upfront payment on the first program, 20% just seems too low. He says he has large expenses with tech support, marketing, and hosting, but the old version of this program is his top money maker by far, and I think the new program will probably sell itself on his site.

Fifty percent each seems like a fair deal to me, but I'm fresh out of school and new to this business so really I have no idea. So does anyone with more experience with this type of thing care to comment? Would I be better off just trying to sell it myself and build my own customer base? Or is 20% of the pie a fair share for a software developer in this situation? Is his asset really that valuable and are his expenses really that costly?