I thought I saw Isaac comment that you could get 5 gold got a completed battle goal if you have all your perks...
It seems this is second...err... third-hand info: (Quote for the lazy, link for attribution)
I asked Isaac directly what he thought about offering some incentive for battle goals for people who already earnt all 18 check marks, and this is what he had to say:
"You are correct that you are expected to be retired by the time you reach 18 checkmarks. Feel free to house-rule it, though, to keep the incentive there. 5 gold sounds reasonable."