Im sure with insurance they simply make it up, so many things about it doesn't make sense, however.
Your post code is a big factor.
If you put another driver on the policy that has a full NCB - like the wife you will find the premium goes down - even though they may never drive it.
My cover, fully comp, 58yrs old, Leicestershire, full NCB for a 3.2 was £350ish, for a 911 3.8S (997.2) £340
Fuel for 3.2 / 3.4, 22 MPG around town 26 to 34 on the motorway
Maintenance - depends which model you go for but budget between £600 to 1500 per yr, higher figure for cars still under warranty, no warranty then use independent specialists for servicing.
Tyres - rears 10 to 15000 miles, fronts 15 to 2000 miles - less obviously if you rag the car often
If you fall in love with the car you will find yourself wanting lots of upgrades - and Porsche parts are not cheap.