Taste does not matter, the key word is PERCEPTION ! If a customer perceives the idea that there getting a great $ 5 shake then it will be in there eyes a great $5 dollar shake. If you are doing your job of selling it as that great only you can supply $5 shake when all the competition is selling is the ordinary, run of the mill $2 shake, it leave a customer with a only 2 choices
#1 go with the only guy that can supply the best
#2 choose one of the ordinary guys from the pack that give the same results as most anyone else
As was pointed out not everyone can afford the extra expense of the $5 shake but it only takes 1 of them to make up for 2-1/2 that can only afford the ordinary "shake"