"Affordable" means different things to different people, but owning one is not nearly as bad as you would expect. It does require a steady in-feed of cash, though, and But yes, it has been noted to be one of the highest maintenance cost cars ever built (if you were to bring it to the dealer for everything). Still, parts are readily available, and there is a phenomenal support community on the Rennlist.com forum that will help you locate what you need for the best possible prices, plus provide you with excellent technical advice, too.
A solid running 1987-1991 928 S4 with auto "only" runs between $10k and $15k to buy. Annual maintenance costs will average out to around $1k - $1.5k per year, but you'll have some big-price-tag years in the mix there. The more you can do yourself, the more reasonable it is. It seems that a LOT of the 928 owners are in it for the wrenching as much as for the driving.
Anyway, for that you get a head-turning classic GT coupe that loves the highways as much as the by-ways, and will easily run up to 150 mph and beyond. It's supremely comfortable, extremely powerful, and pretty classy at that.