Not sure Don, I guess they are reasonably quiet. I'll have the guy asked next time I speak to him.
I am sure you could manufacture and put one together for £400 stealthy, but it would probably take you at least a day to do it, (or a day to get the equopment, install, check, test, etc). So yeh, add labour costs, costs of running your business, and there you have it, the retail price they are charging. After all, they are a successful company that have been in the industry for a few years so they must be doing something right.