If I wanted to buy a sofa, I really prefer to buy the new one, the one that is cheaper but in good quality. When doing an upholstery it will cost the same.I remember when my father in law, he had these single seats that is very old, but still in good condition. He wanted to change the design of it and the foam, so he ask for it how much the shop charge, and it really expensive to do the one chair. He said it is around $200-$300,so I was thinking of buying new sofa instead of redoing the old one. Sometimes we need to evaluate things before we decide, it could be costly or less either the two. If it has a good value, maybe it is worth to keep it and do reupholstering, but if not, then just buy a new one.
On the other side, re-upholstery will save the environment and saving your furniture.