Many posts on NVRAM adapters these days, but I'm surprised the Denikar adapters have only been mentioned a few times in threads. Located @ http://www.denikar.com/ are $16 shipped nvram adapters. They ship out Priority Mail and there are also discounts if ordering multiples. 6264, 6116 or 5101 variants. The 5101 adapters I believe work in Gottlieb System 80s. The website also says the ferroRAM chips come direct from the manufacturer, meaning you know you're getting good stuff.
**NOTE: These aren't my adapters nor do I have any affiliation with the website or creator.
I'm posting this because they are a really great price if they're truly using genuine nvram bought from the manufacturer and because of that definitely worth consideration to anyone looking at their nvram options these days. I think the guy creating them hangs out mostly on the KLOV arcade forums and doesn't really post here.
Of course there are many other people making great nvram products (including myself) and you should buy from whoever you want to support. I'm just bringing attention to another option that really hasn't gotten much press on Pinside.