[mst]

I don't know that much about optical design and costs, but one reference comparison on the market is the Leica 50mm f/1.4 Summilux and the Leica 50mm f/2.0 Summicron APO.  The latter is a full stop slower (i.e. it's optically HALF the size) but is priced 170% higher than former.  (OK, Leica can charge whatever they want but I'm taking the pricing at face value.)

Ah, just saw that, and I just HAVE to comment... mainly because I still don't get all the hype around that 50/2.0 APO

Surprising fact about the Summilux-M 50/1.4 ASPH: it's an APO lens, too Its optical formular is very similar to that of the APO-Summicron 75/2.0, however Leica back then decided to not add APO to the product name of the Summilux... otherwise they'd probably have a hard time now selling the one stop slower lens for double the price

In general, a good APO lens doesn't necessarily need to be expensive. Very likely things get more tricky and difficult and thus expensive with faster glass, but there are relatively fast but still affordable APO lenses out there... just think of the Sigma 150/2.8 for example.

-- Markus