First of all, know that reupholstering an old sofa could end up costing more than you would pay for a new one. It can only be worth it under a few conditions, and here they are:
If it is a Family Heirloom:
If the sofa has been in your family for a while and has been a prized possession with a history and special sentimental value, you may want to hold on to it. There are certain things which are irreplaceable, and you cannot put a monetary value on them.
If it is a Valuable Antique:
When you have a sofa that dates back over a century, and you have invested good money in it, reupholstering it makes sense. This is not something that can be replaced by something brand new. In this case you are actually restoring an antique.
If it has Good Bones:
It makes sense to reupholster if your sofa is very high quality, and the cost of reupholstering it would be less than buying a new sofa of comparable quality. This is hardly ever the case unless it falls under one of the two categories above.
If it Costs Less:
If it is a very good quality sofa, and the cost of reupholstering it comes out less than getting a sofa of comparable value, by all means go for it. You may want to get quotes from two or three different reupholsterers for all fabric and parts, and then weigh it against what you would pay for a new sofa of the same quality.
Remember that there is a difference between re-covering and reupholstering. Re-covering involves just replacing the fabric, and it is more of a cosmetic procedure. Reupholstering could involve replacing the padding and springs, mending the frame or re-staining any wood. As you can see a complete reupholstery job involves a lot of labor and replacement of material, and is more structural in nature.
If you are just tired of the fabric you have right now but don’t want to invest in a new piece of furniture, you may want to look into slipcovers. I would definitely not recommend reupholstering in that event.
In the end it is simply a question of value.
