I have to admit that I last bought one about 18 months ago from Kilsyth, so they have possibly increased in price since. Its on my 200B daily driver and its fixed all the problems and returns better than 30mpg on the highway.
You've got to remember that the Weber primary choke is bigger than the Datto secondary which is one of the reasons that they are so much better
For $20 each, I reckon you got real good value