OK - are you talking a floor flange (a metal plate the toilet bolts down to) or the wax ring that acts as a gasket between the flange and the toilet ?
To replace the wax ring and reinstall the toilet would usually be about $100 - maybe $150 in large cities and high-cost areas - normally a minimum visit charge plus about $5 for the wax ring.
If the flange was corroded so bad it was not holding the mounting bolts, then the cost would depend on how much trouble it was to replace. If it turned right off, with or without the nipple (short piece of pipe it mounts to) then the cost would have been little more than just replacing the wax ring - maybe $10-20 more for the parts. However, if he had to open up the ceiling below or go into the basement or whatever and break it free from there, that would have been maybe an extra half hour to hour.
Not having seen what it took to fix it, and what pipes may have broken when he tried to break it free, I would be reluctant to bad-mouth the plumber, but normally replacing the flange (and wax ring) would be a $250-300, 2 hour range job, so yours is definitely over the norm, to say the least. Beyond that, one would have to know exactly what had to be done (or how long it took) to say if it was "fair" or not.
I presume in all this there was no drywall repair and painting involved in the $535 - if that was included to repair the hole he cut, then you are getting into the reasonable range - at least $400+.