you could search online the private & purebreed rescue sites of this dog (or other comparably priced breeds) within reasonable proximity to you. out here in Cali they always charge tax deductible "donation" fees to cover care, time for evaluating behavior, any training needed to make dogs more adoptable, and generally supporting the rescues. Sometimes the fee starts decreasing when a dog has enough age to be less than first choice by adopters.
some places won't take a large known child biter due to liability. If he bit a child in his own family, he'd have less inhibition toward strange ones. Sounds like poor leadership/supervision by adults. the trick is to prevent acting out and dogs' success at such things, and behavioral patterns.
I think they are lucky to find him a responsible home. you don't have the overhead of Rescues either. but it still requires your own personal work to ensure a safe canine citizen around kids.
Both my rescue dogs charged $250.00 for mixed breeds, absolutely 100% child safe. There's LOTS of available dogs here in Cali, though. If the market is tighter there they might be pricier, and high-end breeds always are, even here.
The fees for private owners is usually to prevent dogs from being sent to test labs and picked up by impulse throwaway adopters.
It doesn't sound like these people would be insulted by getting what they are asking. you could double that if you want, but if this same dog had to go to a rescue, SOME of them might ask for $$ from the relinquishing owner to help fund the work, and then charge adopters too. There is a wide variety of protocols among these places.
Now that you told me he bit their child, it might help to consult with a good trainer about how to start right at getting ahead of this in your home. you never know when unexpected child encounters could occur. good to have big preventative leadership in place.
it's a balance of forming a bond plus obedience with adult dogs.
ps, I highly recommend that you contact that english bulldog rescue and get a referral to their trainer for working with you for the best techniques with his breed. breeds have certain temperament tendencies, and certain approaches work real well, whereas others can spell failure. simple differences do matter.
Being willing to really work at a dogs' adjustment should be worth more to his owners than money in their pocket, IMO.
I don't want to sound tough, but they did fail to raise & handle this dog correctly..
not to be unforgiving. they may have had too much on their hands with a child, but it is the reality, and dog is paying by having to be rehomed.