Ben:
I own an A-36; had an A-55 Baron and now have a P-Baron. I agree with what Lance said above for several reasons, most of which he covered. Your fuel burn and engine usage is very close if not twice the cost of a single. Fuel a little less and engines 2X except, to get to the same speed, in the twin you could run just a little lower power setting.
More systems on top of that. In the A-55 and now in the P-Baron, there is almost always something that needs care. The A-36 just didn't have as much to go wrong in it. Of course, insurance and recurrent training will almost certainly be required for 1MM smooth coverage. Every place you stop, you may expect ramp fees, parking fees, etc. are more for a twin. In a bigger twin, you will also use self serve less. I can't move the twin around by myself on a ramp, and have to be careful where I land and how I park if there are no ground services.
How you maintain the plane will make a difference in this computation, but you strike me as someone that would want everything working and would do a lot of preventive maintenance. To provide you some perspective, my A-36 annual was just over $6,000 this month; almost no airworthiness issues. Did a lot of things that I just wanted to make sure were right: disassembly of ailerons and lubrication of all bearings and inspection; did the wing bolts last year, etc. Anyone that brags about a $1,000 annual on a plane like this, either does a LOT of maintenance during the year, or isn't doing anything more than is required. That doesn't describe me (and I'm willing to speculate Lance, Bruce or the other folks I respect on this board).
An older bird will have more costs simply because systems are older.
Best,
Dave