contact your local cats protection branch to see if they offer vouchers towards the spaying. We got one for a stray we took in last year and it ended up costing only about £26 to have her done. Our little boy goes in on Tuesday and again we got a voucher so he's only going to cost £19. I've heard that the RSPCA offers vouchers as well, but I don't know the details.
Also, I've found the chains are more expensive as they will charge you for every follow up and a full exam for everything. We go to a small vet practice and follow ups are included in the original price and a lot of the time we don't need to see a vet just the nurse which they don't charge for.
My friend's dog had some trouble with one of his claws and her, chain, vet charged for every single follow up. Our big cat had an accident where one of his claws was ripped out and we only paid for the initial visit.
I'm actually a bit annoyed because I've never come in over our insurance deductible with the big one, and he goes to the vet fairly regularly.
I hope Cats Protection or RSPCA can help