Your costs will depend on the details of your operation: Where you live, the surgeon performing the procedure, the length of the operation, etc...
Expect to pay more in NYC, Los Angeles, San Francisco, etc, and also expect to pay a bit more to have your surgery performed by those surgeons with the very best outcomes and reputations (generally speaking).
In experienced hands, the long term outcome of Brazilian Butt Lift Surgery is excellent, and patient satisfaction rates extremely high.
And while I agree with my colleague from New York that in the wrong hands the operation can be regrettable, I would not agree this is necessarily the case or that the fat usually goes away.
In other words, your chances of being happy and safe are going to depend the most on finding someone to do your surgery who has a lot of experience with the technique, and skill in producing lovely outcomes. Look at lots of photos- and be critical. Know that there are some who show photos of others' work, and others who show "after photos" taken while the patient was still in the OR! This will not be representative of the patient's true outcome.