It seems that observatories are partially in the "If you have to ask how much, you can't afford it" category. Thanks for something different!
A lot of it is awfully pricey, but it's a niche market, economy of scale doesn't really figure into the observatory market. Personally, I can't justify the kind of costs to purchase a pre-made dome, which is why I will either build my own dome or roll off. I like builds like this, it does the basic function of keeping the elements off the scope. Our club has an "outhouse" with a big Dob in it, it swings off on a pivot, works well.