How much should poster/talk prizes be spread at a symposium?

I am one of a group of students organising an institute-wide symposium (by and) for PhD students. PhD students will be invited to apply to give a short talk (about 8 spaces in total) or to present a poster (~40 perhaps).

We intend to award prizes for best talk and best poster. It is my view that it is fairer and kinder to have two runners-up in each category and to spread the prize so that more individuals are recognised for their good work and effort. To me, awarding prizes only for 1st place, or 1st and 2nd, ends up assigning reward in a noisy fashion or according to bias in the judging, which ignores the fact that there are many good students doing diverse work at the institute.

My peers did not seem to find this nor the format of judging important - but I feel that spending institute money on rewards in a competition style ought to be done thoughtfully and properly. Having a narrow prize spread acts as a greater incentive in tournament theory; but in my mind the symposium is an infrequent instance of recognition, not incentive, and assigning the prize will be noisy or biased which is smoothed out by spreading the prize.

How much prize spreading do you think is appropriate or optimal? Also, how much monetary value of prize do you think is appropriate in this setting? £100 for first place talk has been suggested. Is this better/worse than a £25/£25/£50 spread? I'd like to hear about people's previous experience with running these competitions and/or winning one of these awards.