You often see syndicates who operate this genre of aircraft (IO520 retract/CSU) charging themselves around $120/hr dry.
So that's not a bad starting figure, and as pointed out by others, usage, looking after it, and helping with maintenance will make a difference.
Putting it on line adds a dimensional twist in the wrong direction, but if managed closely and you are very knowledgeable on the 210 then some of the down side can be mitigated.
A hell of a lot hangs on what condition it is in when you buy in, eng. prop, brakes, tyres, corrosion, paint, mags, alternator, starter, avionics, previous maintenance quaility.
That's why they say if it floats, fly's or ...ah...cooks, rent it.
M