Yep. Insulator. And if you find the antique dealer that pays $40 for these let me know! ($6-8 is more realistic) Very nice piece! The older the rarer. If you find ones without the threads in them they're very old. This one being tapered without the ring around the top crown makes it somewhat rarer, and the green colored are more desirable than clear. If you find an unthreaded earthenware one (like the brown glazed milk/creamer set EVERYONE had in the 70s) you've hit the jackpot...then you might be looking at $40. Of course, it all depends where in the country you're located. Here in New England these are a lot easier to find, since every house with electrical service between 1880-1950 had this/similar style for their electric service in, and of course telephone/telegraph lines. (look up at the side of your pre-1950 construction and you may see this!) In the "younger" parts of the country they may be worth more. Typically I say $2-10 in general in Maine/Massachusetts, with $4 being the average sale of a typical insulator.
I have a box of them in our museum that I discovered that the former gift shop person were selling for .75 cents. on eBay I am selling them for $5.00. I saw one last week sell for over 600.00, but is was from Montreal.