Another vote for cooking something with him from the cookbook. You’ll make memories, not just dinner.
And when I give gifts that have the price printed on them, I take a marker and black out the price. As a child my mother taught me that was polite. (This is one reason why I rarely give gift cards.)
The kid is a cutie, justanothermike is my new Official Dope Crush (I hope your wife won’t mind), and I’ll add another +1 to “cook something from the book with him”.
And I second, third, and fourth getting a Crock Pot for routine weekday meals.
Oh yeah. Walk in the door from work and the house smells like Luau Pork.
what a sweet boy 
I agree with the advice you’ve been given…
I want to say again: SWEET boy!
I was not even remotely “personally offended” by your question. I said I was bothered. Do you see those two things as being the same? They aren’t, by any stretch.
And at the risk of offending you, I’ll explain. You asked the question, and it looks like just about everyone thinks your son did a really sweet thing and suggested you not diminish that by bringing up the money. I was just surprised you didn’t see that for yourself. And to go further out on a limb (sorry), he sounds like a very special kid (I don’t have kids), and I hope you and the Mrs. see that. <I’ll leave now.>
God, I’d been resisting the syren call of the crock pot but daaaaaamn… that even smells good in picture!
You don’t just say you were bothered. You say you’re “quite bothered,” and that’s usually an understatement for being offended. You then go on to chastise him and his wife with a bunch of accusatory rhetorical questions. You then include overly dramatic talk like feeling hurt just thinking about the idea that he had to ask.
If that’s not fairly characterized as “offended,” then what does “offended” mean?