After a long, long, long time, I decided to take the high road sprinkled with really witty zingers. Now, when my sister takes a trip, I tell her, "Oh, I wish we could go but we have to pay rent this month." It helps me, but I don't think anyone else gets the joke.
I wouldn't recommend this approach to anyone. I've seen it done, watched the fallout and have had to listen to the inevitable outcome.
Using "witty" zingers (there's nothing witty about the remark in the example above; it's just snarky) isn't taking the high road. It's being a snide witch. There's no joke here. The person making the remark feels that the target deserves their scorn and they're going to heap it out in spades.
This behavior may make
you (generic you) feel better when you make it, but most of the time it makes others (who don't get the "joke") uncomfortable; they see you as a bitter, petty, angry, jealous person.
The long-term effect is that if you continue with the "witty" zingers, people won't want to be around you anymore and your friends will drift away. They'll feel (rightly so) that if you act that way with your own flesh and blood, then you'll easily act that way with them if/when you ever get angry with them.
And who needs that?
Eventually the "witty" person winds up even more miserable, they can't understand why everyone is judging them so harshly, they explain and excuse their behavior to anyone who will listen (mostly co-workers who can't leave the vicinity or internet bulletin boards) and invest time demanding validation for their behavior so they'll feel vindicated.
To the OP - I sound like a broken record but this question typically resolves many of my issues if/when they come up:
Do I want to be right or do I want to be happy?
This is a case where you're going to have to choose. You may be right in that what your mother is doing is unfair, but in being right you're finding yourself not happy.
If you want to be happy, then you have to decide how you're going to get yourself happy with the situation. I'd recommend removing yourself from all discussions about money and deciding whether or not the time your children spend with their grandmother is worth what you're investing.