About $10-12 each. DON'T skimp and go for cheapy brands, as they may swell up and get stuck in the head. I'd recommend ONLY Motorcraft ZD-11 or the equivalent International #1820697C2, both made by Beru.
Click on the Hard/No Start link in my signature for how to check the GP's from outside the valve covers. If you find any of them open circuit, I'd recommend replacing all 8. It's something that someone with a just a bit of wrenching experience can do.
