I tend to wear out my MM pads quite badly when sanding harder corners ... it tears through the abrasive and the adhesive layer, right into the white pad underneath. I use the set from Hobby Lobby ... around 10 dollars for 6 double-sided pads ... 3200, 3600, 4k, 6k, 8k, 12k.
Right now, I'm on my second set of pads ... the lower grits get worn out first, as I use really light pressure with the highest, to get the finest finish possible. So, my first set is missing the 3200 and 3600, but the higher grits are still usable, so I put them to use with things that are a bit harsher than what I put my new MM pads through, like sanding on stone inlay blanks or metal inlay.
I also picked up some craft foam from Hobby Lobby as well as their sheets of micromesh. It's an interesting pack, starts from 320 grit and goes up to 12,000. I plan to make my own custom sized micromesh pads from them, and see how well they hold up in durability as well as cost compared to just buying the pre-made pads.