An old mat that has been used alot will blunt the blades, it happens at the studio I work at. I finally convinced the boss to get a couple of new mats but the old ones are still being used.
Not sure about the non sealing ones; I have discovered that sometimes buying the cheapest sometimes end up costing me more money as I then I need to replace it. (Pins were my biggest, not wanting to speand the $ on the glass ones, I paid $ twice on cheaper ones, before saying H with it and love love the glass ones, so fine, thin and sharp)!
Maybe you could save up and wait for a sale/coupon and buy a fiskers (my new favorite), or an olfa. long run will be cheaper than replaceing blades all the time.