I'm glad that you did vent, because I believe it may help if you knew how this could have played out.
Parents do not have to contribute anything these days. Nothing. More couples are paying their own way because they can. Ladies are not kept in their homes without education until they are matched with a man who can support her anymore. Women take care of themselves, get an education, and pay their own way. So, the days of the parents paying for everything is waning and should not be expected.
Your MOG doesn't have to give anyone any money. It is thoughtful if she or your FOG to host a rehearsal dinner, but that's it. It is amazing that she gave money for an engagement party. It really isn't proper or polite for anyone to solicit money for a party. No matter who did the soliciting or for what reason, it is just not polite.
All of the parents have been extremely generous and it would be nice to acknowledge this. Please remember that it is you that wants to get married. This is your party, not anyone else's. Why should anyone else pay for it?
I sincerely hope this gives you another way of looking at this so you are not so angry at your MOG.