I'm not so sure that this would be prohibitively expensive. My first approach would be to make a strain gauge package that mounts into the body of the pedal. It would have to sit proud of the pedal's surface to ensure that it bore the whole of the force from the rider's foot, but it would still be protected from extraneous impulses and forces by the body of the pedal. I would try making it to utilize an existing pedal, or maybe a variety of standard pedals, but even if you manufactured a new pedal from scratch, the cost of that new pedal shouldn't be markedly different than any other.

Joe