I did a fair bit of research into running costs, servicing costs and replacement part costs before I bought mine (as a student at the time). The Skylines add up pretty well, especially for an imported car.
Running costs, ie petrol etc, are relative - excellent for the performance levels but not as cheap as say a Barina...
Servicing is pretty normal too (it helps if you trust your mechanic).
Replacement parts have more of an issue with availability than any thing else. It's no trouble getting them, just occasionally have you wait a week for them to be shipped in from OS. This is becoming less of a problem as the cars become more popular and well known. Price, again, is relative - your not going to be able to turn up at Super Cheap and get a Falcadore clutch for five bucks but likewise your not talking big number dollars either.