See this page for current price of 45 mm cutter blades at JHittle:
http://jhittlesewing.funoverload.com/sewing/specials/page_1.html
A note about Blades and mats: Some people are rough on their blades. They use them when they have knicks in them, then press extra hard to make them cut. This results in deep grooves in the mat. I have a friend who tried to see how long she can use a blade and when it is dull, she bears down on it harder. She told me her mat has deep grooves. If you take care of your equipment, it will last a long time.
I recently read somewhere on line that the blades should cut up to 12 layers of fabric. I have never tried that, and probably won't. I cut 4 at a time when I double fold my 45 inch fabic, and make shorter cuts. When I make a few strips from a piece, I put a pin in it to hold the cutting line in place. There have been times when I hurriedly cut another strip and had the fabric turned over and tried to cut with the pin in place. It is a good way to ruin a blade, but sometimes I just forget to check the other side of the folded fabric.