I've sat down many-a-time to work out the running costs of aircraft ownership in an attempt to justify owning an aircraft; yet to be convinced that it's worthwhile unless you do at least 200 hours a year (or cannot get relatively easy access to hire an a/c). I figure a 100-hourly on a (reasonably good) RAA aircraft is going to set you back around $1000, and on a GA aircraft about $2000 - these aren't based on any personal experience but just from talking around the traps with aircraft-owning friends (best case situation, halve these numbers, but average out some "good" 100-hourly's with some bad to get these numbers).
Parking for your steed will cost anywhere from $100/month in the sticks, to $1000/month undercover at some of the bigger GA airfields. There's a heap of variation, so pretty hard to give you a fixed number.
You should also take into consideration interest costs (unless you're fortunate enough to own outright), depreciation (10% a year?), insurance ($3000/yr +), and engine overhaul costs (figure around $10/hr on jab/rotax, $15/hr on a lyc/continental engine).
Putting all this together, I came up with a comparison on a C172 advertised for $70k (with 1000 HTR on the engine) vs a new J230 at $95k:
(sorry about the formatting, couldn't get it to tabulate properly):
C172 J230
Cost $70,400 $95,000
Interest 5.19%
Fuel $1.80 $1.40 (avgas vs. bp ultimate 98)
HTR on Engine 1,000 1,500
Engine Cost $18,000 $12,000
Fixed
Interest $3,654 $4,931
Hangarage $2,000 $2,000
Insurance $5,000 $3,500
Depreciate $7,040 $9,500
$17,694 $19,931
Variable/hour
Fuel $68.40 $35.00
Engine 18 8
100 hourly 20 10
$106 $53
Now look at that on an hourly basis overall:
Hours/year 100 150 200 250
C172 per hour $283 $224 $195 $177
annual $28,334 $33,654 $38,974 $44,294
J230 per hour $252 $186 $153 $133
annual $25,231 $27,881 $30,531 $33,181
I can hire a C172 at $200/hr and a J230 at $145. Which means I need to fly 200-250/hrs a year to justify owning an aircraft. Or, bigger picture, spend $35-45k/year on flying.
Put into these terms, I pick hire over ownership ...
But, and many will agree, owning your own aircraft is something special :-)