Could be a few things, I will tell you this though I have removed and installed a few, and its not fun, take out the back seat, int quarter panel cover, then you need a 9 year old asian girls hands to get in there and work on anything. I personally would send the job to a resto shop with a good service department, the hourly may be a bit more but the techs should be more inclined to do the job. UNless you have a good old school glass guy, but most of these guys are just glue jockeys. And with a resto shop if it is an issue where you may need a little tack weld or something fabbed up, they will be able to do it on sight, vs a glass shop is going to have you come pick it up and send it to a welder...
Just my thoughts on it, I think I personally would pull the seat out, and panel off myself, I am a big donkey so I would take the front seat out too for that side, and maybe if you do that you can see the issue with some light and 10-12 mirrors, lol.. I hate them fking 1/4 windows, once you work on one you will know why post cars do a bit more money, lol... But looking in there yourself you can see what kind of job its going to be and maybe order any parts that are faulty ahead of sending the car and having it laid up...
PS- to answer your question I am thinking 1 hour to get to the problem, 1 hour to fix it, and 1 hour to put it all back together, so depending on their rate, 3 hours+ at $75 is $225 ish, at the low end, double that at the high end...