The general formula being spoken about and linked to ran fairly close to the mark in our case when we expanded not so long ago, however I would go with what gaunty has suggested.
The variable costs are the ones that may catch you out if you do not go in eyes and ears wide open. The planequest formula for the most part does not take into account the variable that there may well be maintanence that has yet to be carried out that has been well hidden or just plain forgotten about. Only an incredibly thorough pre-purchase inspection will cover that, and be prepared to pay a qualified "independant someone" some serious pesos for them to take the required time to delve back many years into the aircraft's log books and Engineering Orders which can take many weeks to do properly. In all reality it is more cost effective to do it while you don't own it, rather than finding out after you do own it (especially when you are spending the sort of $$ that would fund a B200 or similar - as you have made reference to).
Other things to consider are that location specific variables come into the equation. While it may be cheaper to keep the aircraft at a more inland/regional basing due to things like rent/hangarage/landing fees/personnel and insurance costs, the double edge to that is that the cost of maintaining and fuel may well offset any gain in that area, and of course the downtime that comes with waiting on parts to arrive to fix a broken aircraft.
On a side note I'd like to thank the couple of people that helped me gain a more thorough understanding of associated costs involved not so long ago (SuW).
Cheers,
OpsN
