When I did rescue for BRAT - I didn't even charge anthing for expenses. When I"ve done rescue for BCOA or just because a basenji needed a new home - I've only charged anything if there were unique circumstances. I do take a 'deposit' of somewhere between 100 - 200. and place on a 2-3 week trial - if it works I return the money. I sometimes suggest a donation to a rescue organization.
If I was doing LOTS of rescue though and had lots of expenses for health and transport - I would have to charge.
I just laugh that a 'high volumn breeder' that I see her website - charges 3 times what the rest of us reputable breeders charge for her pets - oh well that's how it goes.