Like any new technology, costs come down as time goes by. We've seen it with pocket calculators, PCs, cellfones, you name it. HUDs are still costly, but IMHO, still well worth the investment.
All of SWA's -300, -500, and -700 fleet are equipped (almost 300 aircraft) and the only ones that are not are the 30 or so -200s which are in the twilight of their life here.
Nice thing about HUD (or HGS, the Flight Dynamics version) is that you don't just see benefits as far as CAT II/CAT III approaches are concerned. Not every airport has a CAT II/III approach, but a carrier can usually get reduced CAT I minimums if a HUD is used. At some places, that 2400 RVR gets lowered to 1800 or 1600, which is often enough to make the difference between getting in and diverting.
What I'm waiting for is the cost of APALs to come down (essentially, cruise missile navigation and mapping meets IR imagary, with results projected onto the HUD). That'll supposedly provide CAT III minima to any runway end irrespective of ground equipment installed.
ALL views, opinions expressed are mine ONLY and are NOT representative of those shared by Southwest Airlines Co.