Whats the easiest way to fix a low compression cylinder, and how expensive would it be?

 

How low is it? What is it compared to other cylinders?
Have you determined if it's rings or a valve problem?
You can tell by squirting some oil into the cylinder through the spark plug hole. If your compression loss is through the rings the oil will temporarily seal it and you'll get a higher compression reading. If it's a bad valve the oil will do nothing and your compression reading will stay the same.
If it's a burned valve you will have to pull the head off and get your valves and seats worked on. If so you need to look at your tune because you were running lean to point of causing damage.
In terms of cost it's going to vary depending on your situation. If you've got damaged rings the motor has to come out and you're looking at a rebuild. If it's valves you might be able to get away with just replacing the damaged valve and doing the necessary valve seat work but chances are that if a lean condition was that bad you may have other valves that are close to failure.
Either way it's not going to be cheap.