Is there an available aftermarket .22 magazine for A BRK Ghost that also retains the double-mag snap-on capability (with the magnets) and isn't overpriced like the original one?
Yes there is. I just received four of these magazines from Maple Custom Products in Canada eariler today. Took them out and shot them on the ghost. They work. They can pair up using magnets. Keep in mind they are 3D printed so it is a plastic like material unlike the aluminum magazines from BRK. However, I got the four, paid for shipping and it still cost less then a single BRK magazine. Here's the link. Confirm that it is the one compatible with the ghost.
MAPLE Custom Products magazine Multi-shot magazine for Brocock / BRK Ghost and Daystate Alpha Wolf, Delta Wolf air rifles. Lightweight and super tough. Easy loading & smooth action. 2 magazines will stick back to back for added shot capacity. They will also stick with your factory mag
maplecustomproducts.com
Also note, it has one fewer round. They make it so that the final round is blocked so that the gun won't cycle and shoot when there are no rounds left.
Hope it works for you. Let me know if you find any other alternatives.