There is no average weight. Weight depends on many things, the size when born, the feeding size and frequency, the locality (there are dwarf localities)
Can you post a photo? That'll be the best way to judge if the boa is a healthy size. They should be lean and long with good musculature, in a loaf (square) shape and not round (like a python is healthy when round). The spine should not be a ridge down the back, that is a sign of an underfed boa.