You generally pay more for retail, but you pay for convenience and for the surety that you purchase something that you like and fits you.
As the discussion above noted, whilst there are deals out there, it can be false economy because you may compromise on colour, style or fit for the sake of a cheaper price. Also, you are in no way, shape or form being asked to put your wallet in your mouth, grit your teeth and bend over for $650 for half-canvass from J Crew at retail.
In London, that's the price (at market spot exchange rate and sale price) that I bought my half-canvass suit at retail!
FWIW, the "Good, cheap, fast" maxim is a variant of the Project Management one: quality, cost, time - pick two.