[kls]

They might argue that it is their idea, I argue its mostly in the execution, which you will be a large part of.

Right the mantra around here that ideas are worthless exist for a reason. I would go even further and evaluate what the other person will be doing while you are building the product. If the answer is be the idea guy until it is done then he is going to market it, then I would argue that even 50% is too low. Because you could build it, and then get a sales guy to push it for 10% of the take after it is done. Now if he will be doing business dev and other work while you are working then I would say 50-50 is the right split.