Either your rings are shot, or you are well on the way to a blow headgasket. Either way, you should get this dealt with ASAP before you put the engine at further risk. Valve seals, guides are a definite need to do item. Plugs tell the story. You need to get an assessment of how bad the rings might be if that's the problem. However, unles the car has been totally neglected, the rings are probably not the culprit. Has it ever been run hot?? If so, then it's the headgasket heading south along with the guides and seals. Don't sit on this one for too long. As to cost, maybe Donnie or Steve can give you a ballpark. You're probably looking at about $2,000 if it's the head, valves, gasket...That number is only a guess based on what I have seen posted here. Good Luck..

__________________
Jeff Lawrence
1989 300e
2000 Dodge Grand Caravan SE
No matter what you fix, there will always be something else to fix..
"Warranty" is just another way of postponing the inevitable.