Anything that has a valve capable of metering shot ain't going to be cheap. That being said, nothing else I have seen at Rendezvous, or in the field, works.
These are not a walk in the park to install on a bag, or snake, so they do end up in the parts bins at shops,and traders tables. Most of the time the new owners know how expensive they are, and price them accordingly.
The weight of just a small amount of shot put a tremendous strain on the leather at the neck of the snake, so stitching should be saddle stitch(double needle), and all seams should be welted.
Hungry Horse