First, yes, this is quite typical. Snifters are widely used for beer and generally for two reasons, in my experience.
As you suspect, the bowl shaped glass helps to concentrate the aroma. However, this isn't unique to the snifter as many types of beer glasses incorporate a narrowing throat to provide this effect.
Quantity. With a glass of beer that ...
The first Imperial Stout was made by Thrale's Brewery to export to the Russian royalty, it had high alcoholic content so that it could survive the trip gracefully . Since then Imperial has been used as a prefix to signify beers which are of higher alcohol volume than their normal variants, and luckily they're no longer just for Russian royalty.
The only way to know the price for sure is to sell it and find out the price at that time, preferably at auction.
The only way to price a rare item like this without selling it is to mark it to market. You need to find a number of comparable bottles of special edition beers with similar collectors' markets and use the prices of those bottles to calculate ...
The term "imperial" generally means "strong", as in having a higher ABV. You may also find "Imperial IPAs" (aka IIPAs, or sometimes Double or Triple IPAs), and again, they are boosting the alcohol content. The actual ingredients (hops, or oatmeal, as per your example) do not come into play, except to balance the flavors against the malt.