It costs me about $70 US to frame a 16x20 print in a 20x24 frame, all Light Impressions best board (Westminster board and acid-free corregated backing), a nice metal section frame by Neilson (style #50), and lucite (purchased locally to size) instead of glass.
The 20x24 frame from Light Impressions is a precut size, so it's a bit cheaper that way, and I cut the mattes myself with a Logan cutter that costs about $125 and does a very good job. I use an overmatte, another piece of the same board behind with the print hinged with linen tape, then the backing in the back.
I can probably find cheaper resourses for the matte board and maybe the frames, but I've been lazy about looking, and although L.I. can be inconsistent in shipping service, the product is quite reliable.
If I have a print in a show for $300 - $400 framed, I offer the unframed, and unmounted print for $100 less.