Interesting. If I were in your position I would consider - How much effort I've put into the project (in income opportunity cost) - The capital they are bringing to the new company. Or, how much capital they are the capable of bringing in with their contacts.

I would base determining whether 10% amounts to a fair price on that.

If the site is simple enough to implement, I would also consider how much it would cost them to get someone else to do it. I.e. if I demand 50%, they might say "Thanks, but not thanks. we'll just pay someone else to write this for us.".

10% of something is better than 100% of nothing, but you don't want to be taken advantage of...