What's the financial burden for adopting a child with aids/hiv?

Varies. Some states offer benefits to parents who adopt special needs children. This may include health insurance like title 19. Check with an adoption specialist in your state.
Similar. The noble thing you are trying to do is probably similar to adopting any other child; however this child will have more healthcare needs from the get go(more trips to the doctor, more medications, etc.) which may run up your vehicle expenses but as long as your health insurance plan covers this you should be O.K. Lots of love and support will be the mainstay of what you and the child will need!