Losing my coolant

MY VP Holden Commodore has done 250,000km and was losing coolant from the heater. My mechanic disconnected the heater and blocked off the water supply. That's fine in the summer, but it won't be when winter returns. He tells me it will cost ``up to'' $1000 to repair because it entails removing the dashboard. It requires eight hours' labour to do the job. Given the car's age, is it worth doing and is the quote reasonable?