This article is from the Aviation FAQ, by Geoffrey G. Peck email@example.com with numerous contributions by others.
The general consensus is that if you fly from 200 to 300 hours per
year, the hourly costs for owning an airplane will be about equal to the
hourly costs of renting an equivalent airplane from a local FBO. In a
partnership, evaluate the total flying hours for the aircraft. This
number of hours is required because there are substantial fixed costs
associated with ownership: tiedown, insurance, annual inspections,
taxes, and so on, which must be amortized over flight hours.
Other "rules of thumb" include:
o Operating costs exclusive of capital costs will be 3 to 4 times
the cost of fuel.
o Allocate 2 times the cost of fuel plus an additional 25% for each
10 years since the aircraft was manufactured (this estimate is
from AvWeb, http://www.avweb.com/articles/cost2fly/).
Many people who own aircraft do so not to reduce the cost of flying
but to improve its quality, convenience, and safety. With an
owned aircraft, one can have the equipment one wants in the condition
one wants, and the airplane will (well, mostly) be available when
one wants. There's nothing like deciding the day before a major
holiday weekend "oh, let's go flying to XYZ!"