Well as where I can't tell you how much I can tell you what it is. When you have oil on your plugs it means you are getting "blowbye" which mean oil is seaping past your seals into your cylinder it is either your head gasket is going bad or could be your piston ring or a valve guide. I wont go into detal about it but those would be one of the causes. as far as engine break down if you got a book on the car look it up and it will tell you
Just about a mo. ago I took a compression check and found one cylinder way low. I was looking into my options . Rebuild or replace ? The rebuild is costly , I found a great '82 engine low miles and as clean as a brand new motor inside and out. At my favorate bone yard , complete long block $400.00 End of debate, as it turned out my engine was ok after a valve adjustment but in your case you might start looking arround you might be suprised what you will find. Where do you live ? Gary Portland Ore.