ORIGINAL: Zagzoo
In the usa you can make them for under $15ish. Napa/autozone has the pleated filter (injector filter) for $3, there is a web site called the containerstore for the bottle $.99, fittings $10. Just soldier 3/16'' brasstube to the filter (thru a wheelcollar).
Yes you are quite right, when Sandor at Digitech had his online shop running, you could buy the standard phamesutical bottles from him for about £10 + he also sold the tea bag and all the other bits cheap too, so I don;t know why the BVM ones are so expensive. I bought a couple of bottles from him, as when my Intro went in a couple of years ago, the original BVM bottle got holled, so the parts were removed and I inserted them into a new bottle but I also now have a spare bottle.
Sandor came over to Bickley Jets in 2009, and was selling stock on his stand, he had complete BVM copy air traps for sale for £20, which I purchased one, I now wish I had purchased a few more to use in other models, as people always seem to sell models less the air trap!
Judging by the price of the BVM ones, that must be like leaving a gold nugget in the airframe
Mind you, after seeing Mark S's air traps at Bickley this year, they look great, and at least they look like your getting some value for your money as they are custom made, rather than your paying big $ for something you know is and can be made cheaply.