Seeing as they currently believe it will be cheaper to write it off then repair (by fobbing you off with a lowball payout) then the best thing is to make it clear that you know they are obliged to repair if you want them to, then push for them to repair it. The intention being to negotiate a compromise by accepting a write off value higher than they currently plan to give you but cheaper than it'll cost them to repair. Then repair it yourself on the cheap and pocket the difference.