Well...seems like you've got two issues in play. One person has the more valuable animals...but the other person is doing all the work. Personally, I'd consider a 50/50 split fair...or maybe even a 60/40 favoring the owner of the more valuable animals. 70/30 seems heavy handed...unless you're turning your females over to him (which would be a mistake, as I understand it...a change in environment could put them off of breeding for the season).
Of course the clutches will likely have a variety of animals of different values. The owner of the more valuable animal should get first pick....then you pick one...then they pick one...etc. Whether that is "first pick" from EACH clutch...or first pick from the combined two clutches is up to the two of you to decide.
I agree with the idea of writing up a contract. No matter how much you trust each other, a contract will help make sure there is no misunderstandings in communication. Write up what should happen in every worst-case scenario you can think of. Write up exactly how each clutch will be divided. And write up exactly what the responsibilities are of the person keeping the snakes for breeding and incubation. Then, during the process..DOCUMENT everything! Take pics of locks, keep feeding records, pictures of clutches, etc etc. Breeding loans (especially with expensive animals!) should never be entered into lightly.