When DD and I got rescue guinea-pigs ( this was years ago , 10+ years ) the rehome fee was £10
We checked the price in our local (very good) petshop, they were selling for £20 and P@H £25
So for two adult un-neutered boars we paid £30 (so between the Rescue price and Petshop , if that makes sense ) as we weren't getting the "baby" animal.
When we got a young piglet (male) we paid £20 (because he was a baby , but sadly boars are harder to re-home. All the sows were reserved)
2 ex-breeder sows ( mad as frogs and not handled ) £30 for 2
Castrated boar we paid £50 ( thir vet charged much less for castration but I knew the usual price was £58 from having the piglet castrated )
Then another 2 sows (they were £10 each + we took a big bag of food which was £10ish)
Do they have a suggested price ? Or maybe check the PetShop prices and use that as a guide ? Ask them if they want food donation? Even vegetable if they have rabbits and piggies
Our cats were £80 each , neutered , chipped and vaccinated which is a bargain - but the main concern of Rescue is getting a good home . They have over heads of course but will charge a fee to ensure you are serious too .
Good luck with the gerbils 