It's called "life"
The sunlight readable screen costs probably £10 extra to make. It is a set of polarised layers, AIUI, with some fancy chemicals in there.
They generally sell the screen for an extra £200-300.
The biggest problem is not the £200-300 though. It is the fact that the marketing departments have decided to offer this screen only on devices which are already heavily price-inflated, for vertical markets (warehousing, delivery services, etc) where huge discounts are then given to industrial users who buy say 1000 of them. That's why these things have e.g. barcode scanners as options.
It's a bit like turboprop engines are offered only on pressurised airframes which will always cost £2M+ (well not quite; there is the Caravan etc but you get the idea).
The other problem is that these screens are not very good for watching movies, so Jobs is not going to offer a sunlight readable Ipad.
In the end, a tablet which is not sunlight readable is as useful to a pilot as a chocolate teapot. Probably 99% of airways IFR is done in VMC which in daytime is very bright sunlight.
I print out approach plates, enroute chart sections, etc, but I usually have no printer (or a means of printing) when on the move, so a tablet which can display a large database of plates is the only way to deal with a multiple sequence of diversions.
The other approach is an e-book reader like
these but none of them can run any aviation software; they are purely PDF readers. And since the Ipad cannot run any Europe-relevant aviation software either, it is aiming for the same market. Except it isn't sunlight readable whereas the e-book readers are totally sunlight readable.