That's because you are buying name brand blades. Although not diamond blades I've gone to the dark side and buy my multitool blades for about a buck a pc. Are they as good as name brand. Nope. But they are 10 to 13 times cheaper and last about 1/2 the time. So you use $2 in blades instead of $12 each.
I'd try something like
this to see if it'll work good enough. It likely will.
I did try these EzArc blades and they sucked.
EzArc. Never again.
For our next grout removal job I am going to stay with the grinder but fashion a dust collection attachment for it. Or maybe bite the bullet and buy one of these:
Maxvac
The other problem with those expensive Diablo blades is that they are hard to find locally. My True Value had one, Ace had none, local HD had one. Had to order from the HD an hour away.
I've also come to the realization that so long as I am installing grout of the same color, it's ok if I dont get all the old grout ou of the corners. So long as its in solid shape and I can score it a little bit, the new grout seems to adhere well enough. I also use caulk where the tile changes plane so that hides a little of the old grout