Maintaining a 928 isn't expensive...provided required maintenance is performed on schedule. Neglect a 928 and it'll bite you in the ass...on the way to your wallet.
Of utmost importance is finding an experienced 928 mechanic, not just a Porsche mechanic. You need someone who really knows their way around this particular car.
When shopping a 928, you need to see and drive many cars to better recognize when you have a real keeper. A quick check of mileage and price on Autotrader.com or Cars.com simply won't cut it.
Since this is a Nice Price for this particular car, what would a Nice Price be for my 87 928 S4? Black, Black, 5-spd, 30,500 miles, no rear air, all stock and original except for window tint and stereo head unit?