@wiltw (post #23) can give more details as he is (one of) the people I was mentioning who bought this:
Price is for one adapter only. Use of 386 / SR43W Silver Oxide battery (1.55 V) to.
www.ebay.com
Here is what I bought and using productively:
Our mercury battery adapters are the perfect solution for vintage collector cameras and equipment that were designed for use with mercury batteries.
There are others on ebay that ship from Japan and priced somewhere in between. Ironically, they all look the same... and maybe they really are all the same. As long as one doesn't make the mistake of buying the much less expensive adapters that do not do voltage reduction. As mentioned above, shipping from wherever to wherever is very costly, especially for small items.
For me these adapters are worth the price as they enable use of old gear without fiddling with modifications or short-lived zinc-air cells. I use them in a Sekonic Luna Pro, Richol Singlex TLS (my most hated camera but it is an heirloom so I use it anyway), and in the past in Canonet. They provide good and reliable service but, as you say... at a cost.