I would have just said, “We thought it was worth it for them to have fun at the best camp we could find!”
On a related note, my relatives were talking on the phone about softball training for girls in the family.
The one said, “There is a softball team 30 miles from here and they wanted over $200 per season! Can you imagine spending that much money just to be on a team!?”
The other said, “Well, we have been spending $300 per season since she was 7 years old - not including the cost of going to all of those tournaments and the overnight stays and the flights/drive to get there! Then again, that training is why she has just gotten a full scholarship to a top ten university for softball.”
The fact that you paid a premium to send your kid to a great camp speaks volumes - some parents would have bought a new flat screen TV or whatever - you most certainly splurged in the right manner, for the right reasons. Kudos to you!
This really varies a lot by social class. In my the working-class neighborhood where I grew up, “how much did that cost ya?” was absolutely a normal part of conversation. Everyone was always looking for ways to stretch their dollar, and it was considered normal to share your financial tips, triumphs, and worries.
I think there is an element of the “anxious middle class” at play here. Because there is such a tendency to identify someone’s value as a person with their salary, it become rude to discuss finance less you implicate that one party is more “valuable” than the other. There may also be some of the residual stuff stemming from the old rich/new rich paradigm, where it’s still consider slightly distasteful to have to earn your money from working.