You can sharpen rotary cutter blades, easily and cheaply. It won't get rid of burrs and nicks caused by rolling it over a pin but it'll sharpen a blunt blade nicely.
Take a piece of aluminium kitchen foil (about A4 size) and fold it multiple times so it's a thickish wodge of foil - four to six layers, probably.
Place on your cutting mat and cut into it with the rotary cutter and blunt blade. After about a dozen swipes, your blade will be nice and sharp and the foil will be shredded (AND DEADLY - this is the bit I always cut myself on so dispose of VERY carefully).
I've had the same blade in my cutter for a year and a half now. Saved myself a fortune!