I suggest that you call FX and ask them. I’m sure they’ll give you an estimate. I once had a Benjamin Marauder resealed by Baker Airguns. It was close to $100 as far as I remember.
I would strongly suggest that you learn how to do it yourself. You can find a diagram and a list of o rings through the FX website. The Boss is not a difficult gun to work on and if you have some basic hand tools you should be able to figure it out. Look at the diagrams, be careful removing and installing the o rings (don’t cut or scratch!), use a little silicone grease, and you’ll be fine.