I agree. I would just leave it. We try to preserve the internal bore when we anodize barrel backs so the integrity of the bore is not compromised. If you are going to anodize something, spend the money to get one of your other barrels anodized so everything matches!
The numbers you were quoted are not high. If you ask me, they are low. There is a TON of work involved in anodizing, not to mention headaches. To anodize a complete gun you are looking at at least 4 hours of time. The guns need to be cleaned (they always come caked with paint and grease), they need to be stripped, they need to be re-finished, they need to be cleaned again, they need to be de-oxidized, they need to be anodized, then they need to be dyed, and finally, they need to be sealed. To top that all off, not everyone has the skills needed to successfully anodize.
Unfortunately, if the threads in the guns body are toast, You are going to need a new body. If its just the threads on the barrel, a new barrel is needed. Depending on the severity of the damage, you might be able to use a helicoil, but I wouldn't count on it.
I only brought up the temperature thing because you mentioned self welding paintball guns
It turns into a liquid at about 650*C. or about 1200*F
My numbers are based on the material being 6061-T6 Aluminum, which is what 95% of paintball parts are made from. Im sure each different alloy has slightly different numbers, but they are all within the same ball park.