Curious what the co-founder does, is he the idea guy, or is he the dude that will start thinking about marketing and sales sometime after the product is done. Is he getting a salary from the primary investor or is your salary the only expense.

If your salary is the only expense, then you are investing $65k a year as well since you are working for half normal salary in order to design and build the entire product that is the basis of the company from scratch. Given this, I'd estimate your contribution as either 50% or 100% of the company value depending on whether or not you are getting this salary from the investor, aka founder man.

Not that you'll get this of course, unless you had previously chosen to go for 100% by simply developing it on your own after asking yourself what the other guys were contributing (not much).

But it's too late for all this, you've signed a contract, so you should follow through, be happy with it, and do a good job. Thinking you were screwed won't do anyone any good, but if a job appears that offers more than $65k, if it was me in your position I wouldn't feel all that bad about taking it.