Tough to say.
You can put 2 rings side by side that to me look pretty similar but they might be very different prices depending on the stones. I'm no jeweller.
I think most custom work tends to be 18K gold and I think most store stock is 14k, so that factors in too.
Compared to most of my friends getting married around the same time I paid more. I'm going to guess 25% more. But I'm not sure how much of that was for 'custom' and how much was materials. It could also vary a lot store to store.
Guess I'm not much help
