On Monday, I had 10 16x20 mats cut to my required size window at the local Hobby Lobby for a total of $78.19 which included tax. These are not high-end boards but they are sufficient for short term display at home or at my office. I had contacted a competitor to Hobby Lobby the day before and was given the price of $130.86 for cutting 10 mats on the same board.
I buy pre-assembled aluminum frames with glass installed. I haven't bought any in so long I don't remember the price--I bought quite a few at one time so they must have been pretty cheap.