$45-$50 /hr, whether ground, single, multi, doesn't matter.
Not trying to pick on Rotorhead1, but I've never understood this multi-tiered rate schedule for instruction time. Yeah, many FBO's and flight schools use this as a way to increase the hourly pay for more "senior" instructors, but, free-lance, an hour of my time giving instruction is an hour of my time, doesn't matter if it's a twin turboprop, or somebody wants me to show their 3-yr old how to drive a pedal-powered Stearman toy, it all costs the same. (Just my 1/2 cent!)