Hmm that's a tough one. I totally see your dilemma and understand that your question is not about the absolute rates involved but rather the rate increase seemingly precipitated by you telling her how much someone else would charge. I understand why the day care providers on here are responding to your questions the way they are, of course either rate is incredibly reasonable. But that is not your question.
I guess it's hard to imagine how you could bring it up now, after the fact, without it turning awkward/ugly. On the other hand, it would always bug me if I didn't know why she had decided to change the rates and the further away you get from the incident the more exponentially unlikely it is you will ever find out the answer, even if you ask.
So, if you (or your husband) really want to know, perhaps try the following. Assuming you are going with it either way, the next time you see her in person (I wouldn't do this in a call, email or text) you might say something like " I was reviewing the childcare budget with my husband, and he wondered why watching "johnny" 1 hr a day was costing us $25 instead of the $15 you had originally quoted. I told him that I didn't know as the first time we talked it was going to be $15 but when I made the final arrangements with you it was raised to $25. What should I tell him for an explanation?" Something like that might be less confrontational since you are "blaming" your husband for wanting to know.
I think that if you make your husband himself ask it will be even more awkward BUT if he wants to know and you truly do not, then that's also an option.
Sorry to not offer much more than a wishy-washy opinion, but I wanted to let you know I "get" your question.