That connection from steel to plastic needs to be water tight. Installing 7' of top casing is tricky if you don't pull the pump first. You will also need to change out the part of the pitless that is on the top of the first joint of pipe. That is also hard to do while holding the pump in the well. I know it sounds high but, I think the guy is giving you a fair price. You might want to just do what you can of the job, and let him make the connections. If you drop something, it won't be cheep.