A few people mentioned that they may be charging more because it's more "annoying" to build it left handed. "Annoying" means that it takes longer to do, and therefore costs more in labor. You're paying the high price because you believe that their work is worth the cost, so them having to spend extra time building your instrument should also be worth the cost.
If you contracted someone to do work on your house and they had a standard price for most jobs, but your house had something different that they didn't usually deal with, and it took longer to do the work, wouldn't you expect to pay more?
If you contracted someone to do work on your house and they had a standard price for most jobs, but your house had something different that they didn't usually deal with, and it took longer to do the work, wouldn't you expect to pay more?