Whenever I receive a gift of any kind, I NEVER ask how much it cost. That is just tacky.
I have a family member who always asks how much I paid for this and how much I paid for that and what did this other thing cost and where did I buy it. I got so fed up with the questions, that I came up with the same answer every single time: "It cost more than $25.00 and less than $5,000.00". LOL
Because you do not want to tell her, then don't. If she should ask, then let her know that it is none of her concern because no matter how much it cost, you are happy to be spending your life with her.