Adam,
I flew a high-end single and a pressurized twin side-by-side for 15 years and my experience is that the twin costs at least four times more than the single, if you are lucky. I don't take any depreciation into the calculation. Both aircraft were bought new.
Twins loose an enormous amount of value, compare the second-hand prices for twins (Barons and Bonanzas are nearly identical except for the engines).
Of course, sitting there at FL240, in comfort, is nice and has its price.