Hello Kirstin, A tear off will run about $10,000 compared to at least $50,000 for a couple dormers.
If you can get away with one dormer, you can maybe get it done for about $30,000.
If you were actually tearing out the structure with a tear off, I see your reasoning. However, a tear-off usually just means that they are tearing off all of the shingles not the actual roof.
The labor involved with removing portions of your roof and rebuilding can be extensive (not to mention new drywall, insulation, and electrical on the inside.
I'm not trying to talk you out of it. -- If you need the space and will use use it, do it. -- However, if you are trying to increase the value of your home, you will only get about 80% back on what you spend to do the dormers.