This has been an interesting thread, my standard response when people questioned our £30 adoption fee was that a female would cost you £42 to be spayed at the vets I use, and neuters were included in our adoption fee, never mind the flea/worm treatments (we didn't vacc or chip due to costs). The rescue I volunteer for now charge £60, but that includes neuter, flea, worm, chip and vaccine. One of the rescues in my area did used to have details of hwo much things cost in both their reception area and on the website, although they have changed their website now and I can't find it anymore - I think some of the costs were quite high though, and might have had the opposite effect
I did sometimes think that a compulsory donation prevented anyone giving more, and also sometimes a good home was more important than the donation.
While I have paid for very few of the cats I have had through a rescue, I have always done other things such as donating food or paying standing orders/membership fees (except for the last two, which I fostered/fundraised/did admin for)
I'm still on the shelf on compulsary insurance, partly because there are some poor insurance companies so some people would still be left without cover, and partly for the people who end up with a stray with health issues.