So sad to hear about your little hoggy but at least it’s no longer suffering.
I recently had to do the same with a little one found in my garden during the day. So upsetting.
The vet I use does not charge me for any treatment or putting to sleep.
There is a box for donations.
There are 2 vets locallyish to me that do not charge for treating wildlife.
Perhaps you could contact a local carer or wildlife hospital and see who they use and if they charge.