Cost in my area: about $70 (central IN)
Frequency: I think that depends upon how much you use your machine. I have one machine that I use the most and I try to take it in about once a year. I keep it cleaned out with canned air. I have a 2nd machine that I do not use as often and I only take it about every other year.
I would think the kind of fabric you use alot of might make a difference, as well. I take my machine to the LQS where I purchased it. I love my dealer and all the people in that store!