Why don't you say...

"MrsHold, I know you researched this and believe it may be something you really want for DS this summer. I know you know our budget cannot affor it and that you can earn the money to pay for it. I support you in achieving this for our family."

Would you consider adding...

"I am continually perplexed when you bring something to me, like this, knowing it is outside the scope of what we can do. Feels to me like you want me to feel like a failure, to feel bad, knowing I want you and DS to be happy and successful."

And this is me, believing you like MrsHold to feel delighted...that it pleases you when she does. That's what I believe of you. Within moments, I think you resent her feeling happy...doesn't take away the initial pleasure for you.