Nta.

If she didn't want to get it she shouldn't have promised. If she promised, then found out half way through she couldn't afford it like she thought she was able to, she should have discussed it with you so that you guys discuss about another gift or solution or whatnot.

You mention you pay all the bills/rent. Therefore I'm assuming she's pays for groceries and food. Which means you've paid her expenses so she can specially save up for your Christmas gift. And in the end, she didn't buy it.

And honestly, the way you told her what you thought about it was good. And yeah, is the thought that counts, which was why she shouldn't have promised you the expensive gift, have you paid her expenses so she can save up for the gift if its the thought that counts. And I'm wondering where the money she saved up went as well.