mine costs very little to keep running..
but here is the rub. when something needs doing i do it, not let it fester and run into bigger bucks from putting it off.
sure, things will wear as on anything mechanical and i've also spent a little engery and time into getting the car up to my own high standards.
the box is actaully a piece of p1ss to work on, when i first started out with it i was very much shy of getting my hands dirty because it is a 'porsche'.
now i use nitrile gloves.
seriously though, there is a whole host of maintenance that you can do yourself brake discs (rotors) pads drop links etc, you just need a standard socket and spanner set of decent quality and perhaps some more tools along the way like a meter long breaker bar and a torque wrench, oh and some torx bits for the socket set..
i drop my front bumper/valence off every year and clear out the debris to prevent the rads/condesors rotting out.. it takes less than 15 mins...
just changed the spark plug tubes, using the data from this forum and the transom plug method (tip when you do the rubber bung up keep turning it with some grips and your feel a pop as the rubber seals slip loose and the tube slides straight out. other wise if you try to pull the tube out the plug slides out the tube) now i've figuered that out i could redo a set in about 30 mins.
A BIG tip for doing it yourself is clean off bolt / torx heads before getting into it and use the correct size spanner / torx / socket rounding heads off will just leave you having to get bolts machined out. also use a breaker bar to break the seal on a bolt dont use a short socket and just keep tugging, you'll end up rounding heads off.
i've only ended up doing work myself as i've fed up with 'specialists' charging the earth for frankly crap work and i know that if i do it myself, it will be done to the standard i expect.
tyres shouldn't wear out too quickly i've had my p-zero rossos on for close to 24 months now and they still have more than 6.5mm, but then i make sure the alignment is done regularly,
poor alignment eats tyres.
my sister on her volvo xc-90 has gone through 2 sets of tyres in the past year and i keep telling her to get the alignment done. she's done more on tyres on her volvo in one year that i have spent on the porsche in 5...
again, it's keeping on top of things, when you get the boxster tip top keep it that way, it will cost you less to run and maintain its value..
as one poster said previously there are cheap boxsters, but they are cheap beacuse they need money thrown at them.. no matter the mileage a well maintained boxster has considerable value where as one that hasnt doesnt.
if you get stuck, or need help there are some very, very knowledgable people here that will be more than willing to help you out.
keep the faith.