I'd look at the actual costs. The cost of the boom lift (or however you're getting access), assuming you're renting, including a markup of 20-30 percent of that cost.

Then triple the man-hours it'd take you at normal height. It's going to take time to get up to that height, and you'll work less efficiently--wearing a positioning belt and/or fall arrest harness.

I'd say triple the normal cost would be 'bout right.

Cliff