A typical servicing can add $100 - $200 to the cost depending on what needs to be done. If the machine just needed basic cleaning and oiling then it's on the lower end, but if it needed electrical work too (and many do) then it's going to be more.
My husband is very thorough when he goes over one, and it takes him 4-5 hours if it doesn't need electrical work. He basically strips everything down and cleans any gunk off and polishes and re-oils. Then he adjusts tension/timing as needed and then he gives it to me for a good sewing trial. I usually make up a couple of nice test blocks to send with the machine.
I'll add that there are a lot of variations in how nice the machines sew. Usually those that are more worn end up sewing much smoother as they are well broken in compared to those that have prestine decals and look like they've never been used. Often when you get a repaint, you'll be getting a machine that was well broke in, and then it gets a very good cleaning - and you end up with a beautiful machine that sews very nice.
the last one my DH did was decent, the one before that I nicknamed Sweetheart as it was one of the nicest I've ever sewn on. Unfortunately it wasn't one of mine.
