I buy all my precut matts through FramingSupplies.com. Even my biggest matts, like 32x38, are $10.50 with a Crescent buffered board. Now, get an account with your local frame supply company and have them deliver the acid-free foam core and mounting corners, tape, etc. at wholesale prices.
Then, choose a narrow moulding to keep costs down. Wide mouldings are out. Narrow frames give you a nice finished edge without overwhelming the image.