I contract out to a few hedge funds for writing trading systems. I have my main product and roll updates out every few months.

if a fund wants a couple of hours a month they pay a flat fee per month to me. If the time needed is less than 5 hours then no problem. If its more then I bill on a daily basis.

Rate depends on who gets final ownership of the code, ie can I resell it to another fund or is it theirs.