Show him salaries for similar jobs, and say, "this is what the market is for people who are talented at this, and it's probably only going to go up. You get what you pay for, just like you would if you hired an oil field engineer, or a civil engineer for building a bridge."

Qualify your initial estimates as having been initial, and ask if he wants more accurate revisions of them, based on current market prices and a need forflexibility if there are issues or problems that lead to overruns. Then overestimate, and use savvy business skills to "save" the difference, if possible.