We have two American Bulldogs, both adults and a selection of Staffies/Staffie X's from the age of about 7 months upwards. Our minimum donation is £150 which covers neutering/spaying, micro chipping, vaccinations, flea/worm treatments and where necessary dental. I doubt you will find a rescue that doesn't ask for a donation to cover some of this cost and help them continue their work.
There is an American Bull Dog Rescue site at:
American Bulldog Rescue - Home