It's exactly why she's wealthy.
YABU for being surprised. I also think it's a good thing in many ways that your DS doesn't link 'high quality' with 'lots of money' because the two don't always go together.
Put it this way, if your mum was really poor, you'd never expect gifts, or rather any gift would be a bonus... why should it be a given that the wealthy spend their cash on more pricey gifts?
If someone is very wealthy and they share their wealth somehow, through charity or making a difference, or employing someone or putting them through college, then all respect to them. I DO respect them more. But I would never judge someone who gave shit gifts and were also wealthy.
If I win the lotto tomorrow the £10 per person agreement would still stand amongst family and friends (aside from the drunken debts of houses I've promised them). That doesn't make me tight and even if it did, it's not anyone's right to question it.
It would be 'lovely' if really rich people all tried to make a difference with their spare cash, but they don't HAVE to and I wouldn't judge them if they didn't.
Your mum IBVU for keeping gifts (cash or not, rich or not) that were not for her, it's theft really. The money was for your DS not your mother. On that count alone you win hands down, that's not 'tight' it's downright dishonest.