Reel repair

I have 6 Penn International 50TW reels. I've noticed that the drags are not
as smooth as they should be and it's been recommended that I have them "Gone
Through". I assume that this will entail a drag replacement and cleaning
and lube job etc. I've never done it before, and I'm wary of sending them
in and asking someone to "go through" them because it seems that every time
you send something in for repair and don't know EXACTLY what needs to be
done, you end up with a HUGE bill that you weren't expecting.

Has anyone had any experience with this and what am I realistically looking
to spend to have my reels serviced?

Thanks in advance!

Gary