Hi there
It's normal for you to feel this way, but you need to remember that despite this feeling of inadequacy, your partner will just be happy that he was able to please you, and he more than likely won't mind that your present to him was cheaper.
I think you should just tell him briefly how you feel -- something along the lines of "I love your present to me, it's perfect, blah blah blah, and I feel rubbish for having given you *this*" He will almost certainly be flattered and say that he really doesn't mind, and that you shouldn't worry.
Good luck!