Yes, you absolutely can get it installed without cerakote. All cerakote is for is a tougher finish and aesthetics, A muzzle brake has nothing to do with it. If he doesn't want to install just the brake without cerakoting, then I guess he doesn't want your business. You are the customer, not him. Not saying cerakoting is a bad idea, but if the customer doesn't want it, and it isn't required to maintain the integrity of the rifle, then don't do it.
As far as pricing, that seems on the high end, but still reasonable, if you provide the brake. If that includes the brake, that isn't bad. For one color cerakote of barreled action prices seem to range from $125-$225, averaging around $175ish. Brake installations run $50-$150. So that would make price of brake installation and cerakoting from $175-$375. Brakes run from $50-$250 depending on what you go with, so that would raise the price to $225-$625.