The most I've paid for a tree was $375 for a 45 gallon jabo. Not sure if 375 worth of jabo fruit have been eaten off it yet, but it's nice to have a larger tree that keeps producing. For a slow growing jabo, buying larger makes more sense.
Agree that smaller trees, and certainly cheaper, would be better in your situation. Maybe you can find something reasonable in the 15 gallon range. Not sure about your area, but mangoes and avos grow fast here.