It depends what kind of gun. But look for about 20$ - 45$ an hour for mill work and it will require at least 2 hours manly for prep and programing then it the milling witch adds a good 2 - 6 hours depending on how experinced the machinest is. I mean hey my dad has ben working my 99 cocker for 1 month off and on redesing it (where trying to make it real bad ***) . But hey thats my dad. Back to the subject youll probaly find really exstensive milling work for around 250$ -350$ and minor milling work for around 50$ - 100$.
hoped that helped