You won't really get away with £500 regularly on any model of Porsche of any age.
My 944t has just ticked over 276k miles which makes it virtually unsaleable at any sort of sensible price but I use it daily, it's very reliable and I have a good indy to look after it.
It cost a large sum to have rust cut out of the rear end a couple of years ago and I can see a little more at the front that needs attention. Usual maintenance and upkeep is about £1000 a year
I look at it as depreciation free but running cost heavy. It's insured for more than I think it's worth but not necessarily more than it might cost to replace it, if that makes sense.