I'd not really considered it, to be honest. Although, for a start, I wouldn't know where to begin with making it members-only!
But the costs of the site are neglible - hosting is only a few pounds a year, and although I do spend a lot on buying in the glass to illustrate the site (and nearly as much on materials to research the content) I rarely buy any glass that I can't sell at a profit through my shop or on eBay (and the research materials also aid those sales).
Plus... I enjoy doing the site. It's my replacement for pubs, clubs and a night-time social life now that I'm a regular sickly-soul - and it's a LOT cheaper!
Finally, I've always kind of viewed it as a long-term marketing campaign - if I can broaden the awareness of Holmegaard/Kastrup glass outside of Scandinavia, then it can eventually also raise the value - which, again, bodes well for my shop. Okay, so that one's a bit of a long-shot..... but you never know!
So as long as I've got enough pennies going spare to feed my glass habit and cover my frugal living costs, I'm more than content to keep the site free for all.