while it is true that wide angle is "seemingly" expensive on a digital, your reference of a 28mm is not the best either, as they were cheap on film,
Go to a 24mm and it gets different and 20mm is worse still.
I never ever fould 24mm wide enough on film, but the options to go wider were too expensive, and the older lenses (available at the time) were in excess of $400 new, and that was 20 years ago.
I paid $550 in 2006 for the sigma 10-20 and while it is F4-5.6 it equates to a 15mm - 30 film lens, and, although it is only APS-C rated it is full frame from 13mm on my PZ-1. Now 13mm on full frame is really really wide.
The point is, yes things are expensive, but in terms of comparing purchase price new $ from the 1970's and 1980's to today, lenses and cameras are a bargin today.
and just try to get an SMC 15mm today for < 400
also, if you thing we have it bad, look at the 4/3 bodies where the equivelent to the sigma 10-20 from Olympus (a 7-14mm) costs $1500 or more.
and who says a fast 15mm from canon is cheap, let alone the extra you pay just to have the FF body,
everything is a trade off.