I paid £20.45 for my airzound (was on Discount but the postage added up). This was the cheapest that I could find for the Fisher one. I didn't know about the other company, is it new?
This seems like a good deal:
http://www.chainreactioncycles.com/Models.aspx?ModelID=20105
It's £5 less than I paid. I know that the Fisher's one costs the trade £12/13 to buy, no idea about the other company but it should give you some idea of what you'd have to pay.
I think anything under £20 for an airzound seems to be reasonably good value, or was when I bought mine.
I would say that you may need to buy longer cable ties and you may have some other problems fitting it, but when fitted it is good. For my road bike I'll have to make a new bracket as the handlebars are too thick.