Dufur is absolutely correct and sorry if that wasn't clear. 8-10k is my guess at the cost for two "clean" phase inspections done simultaneously. Add in other stuff that breaks since the last inspections and it will start to ramp up pretty quick. Battery is ~3k, a set of main tires is 2k+...nothing's cheap. Actually our lowest bill for a major trip to the shop was to repair a bird strike to the leading edge. Reskinned a portion, new de-ice boot, and out the door for 12k. Couldn't believe that one, but I bet the insurance company was happy.
I don't know of any STC to put 850shp on an F90. You can get the Blackhawk -135As that they claim will give you 10-15 knots in the flight levels, but they are still rated at 750 shp. The -41/-42As are the 850shp engines on the 200 and are a bigger engine with an additional power turbine. Again, I've never seen a way to put them on an F.
We're probably pushing 900-1000 pph on initial climb, but that falls off pretty rapidly to maybe 600-650 pph by 14-15k. Unless I'm heavy, I still have 2000fpm of climb at 15k, so unless weather or airspace dictates otherwise, I usually punch it on up to the flight levels to save a little gas if the trip is anything longer than about 120nm.
I've never seen the 65-70 gph your friend stated without going to FL260+ or pulling the go-fast levers back. The 600 pounds for 120nm trip at 10k seems crazy high. That would be ~1200pph. Are you sure he retracted the gear and flaps?