Neither Riley or Skye were chipped, vaccinated or neutered when we adopted them from PAWSS, but I was still more than happy to pay £35 for each of them (the suggested is £20, if they haven't had anything done). As far as I'm concerned, part of it is about being a responsible cat owner, but more than that it is about bearing in mind all the amazing work these rescues do, and the fact that without them, you wouldn't have met an fallen in love with the furbabe you adopt. Frankly, that's beyond price.
I would also add that by the time we had each of our babes chipped, neutered, vaccinated, de-flead, wormed and insured, it came to more than £50 each - it seems not only reasonable but positively cheap!