I had an idea similar to this where when filing for a patent you include an estimate of R&D costs. If the patent is awarded then its lifetime is related to how long it takes to get a reasonable payout on the R&D investment.

This way big pharmaceuticals put a couple million into a new drug and they get to profit for a few years. An Apple engineer makes slide to unlock in a couple days and given the number of iphones sold the patent is over quite quickly.

tl;dr a salary cap for patents based on R&D cost