I get 14 - 16 GPH in my Six in cruise, which works out to about 140 KTAS. It'll burn 23 GPH on the climb out, and 10 - 12 GPH back down unless you just nose it over and plow down at the top of the green arc. I pay closer to $80/hr for gas than $100 most of the time.
If I bothered to account for the full cost of mine excluding fuel, I'd probably figure about $25/hr just for the engine reserve, $10 - $20/hr for insurance (depending on how many hours it flew a year), $30/hr for annual and random maintenance, plus a few more for oil changes, tie down fees, GPS database updates, and an emergency fund. Plus, depending on how the deal was structured and how many hours it was being flown, I'd have to start doing 100 hour inspections too, which would probably be about $12 / hour.
$100/hr dry is about fair for that aircraft, at the end of the day, it depends on how much you're going to fly and take advantage of both scheduling and the capabilities of the Six, compared to whatever alternatives are around where you are.
Hobbs and tach conversion depends on how you fly. On mine, 75% power usually means I'm doing 2400 RPM above about 7000 ft, the difference between Hobbs and Tach for me is about 5% on the last 25 hours or so of multi-hour cross country flights. If you spend a lot of time on short flights where you're spending a a fair chunk of time on the ground with the prop spinning, in the pattern, or down low at 2200 RPM, you'd probably see closer to 20% more on the Hobbs than the tach.