jen: next time go to a different vet that doesn't know you and say you 'found' the poor mouse somewhere. The probably will not charge you to euthanize a 'stray.'
I used to live near a vet that would euthanize or even try to treat stray/wild critters. Once I found a fledgling starling with it's EYE hanging out cause some crows had got at it. I held it bawling, waiting for the poor thing to die but it didn't...so I walked down to that vet and he euthanized the poor thing.