***Special EDIT When initially reading your post I read it as that these were Pricey and not simply NICE, Please forgive the rant on the price...***
Yes they are...when you consider what you are getting for the money... Realistically a rolled flange and cap similar to those on the peanut tank sell for $25 or more made out of aluminum. IE this one:
Aluminum Neck with Flange and Cap
The thing is that if you are considering building a custom tank quality parts cost money. Besides Bung King also as the female inlets for mounting the tank and also the petcocks for the fuel outlets that are standardized to industry specs making your job easier. I have 2 more inlets on the way for my tanks and have used them in the past and unless you decide to buy peanut tanks (which would be realistically more expensive) and canabalize them then you need a source like this one.