Well first off, who makes RLPL shirts? Last I heard it was Lorenzini
Second, RL markets RLPL to a certain wealth of people, it doesn't necessarily mean that the shirts are really THAT much better than say Brioni's. Solid construction, great fabric, and fits a certain type of man.
In general, as quality increases, price does not necessarily increase at a linear rate. RLPL isn't stocked at the same level as BB or PRL is either. Whereas BB recycles clothing often, RLPL is producing new and unique designs per season, per year- to a certain degree it is nearly entirely different per year.
Third main item, the fact is RL does not necessarily do the best job at planning stock and price, that's why they have huge sales, to push out items and turn over inventory. When they put up their shirts at $400, it is made up of a lot of mark up, and meant for those who, frankly, can care less about price. People are paying for the 'design', 'exclusivity' and brand image associated with RLPL.
In the end it would probably cost less to have bespoke shirts made for you at your behest.
My current favorite example of this is their Duffle Coat
, That's $3k for a duffle coat...one of the best duffle coats are made by Gloverall and are under $800 - another good one is 'Original Montgomery' and those can be found for even cheaper. RLPL is basically exploiting their ability to overcharge their customers for their 'style', 'exclusivity', 'brand image' and personal luxury - which they have every right to do, and they might as well keep doing it, especially if people keep buying it. (note: the Gloverall's and the RLPLs both share a similar ratio of Wool/Nylon, the Original Montgomery has a lower count of wool)
-This all coming from a RL fanboy, mind you...