I agree with CSK. I would be tempted to have a motorcycle place do them. $300 is just stupid. A good tech may be able to get close on the oil weight just by examining what is currently in them. There is a guy here who does offroad stuff and he quoted me $70 to change the shim pack for an air shock that had both nitrogen and oil. He had a machine that would show the difference in dampening rates before and after.