I personally like to keep a few of our horn making artists in business, so I have always bought the custom stuff. I may have paid a bit more of a premium to begin with but have never lost a dime on any that I have resold.
There are so many top notch custom made powder horns on the market these days and having a good friend that is a "horner", I know how much he struggles with all of the competition.
I have also made a few of my own powder horns and in doing so, learned to appreciate all of the work that is involved. I did OK as for making a functional horn but found buying from a skilled artist met my needs for great craftsmanship (eye candy) that I proudly carry everywhere my rifle accompanies me!