Some A 28mm photos:
SMC Pentax A 28mm f/2.8 - a set on Flickr
I like my A28mm, it's a nice general use focal length on digital. Mine seems to sharpen up quite a bit going from f/2.8 to f/3.5. Somewhere around $80 to $100 would be pretty reasonable in good condition, maybe a little more if you're impatient and decide you like it.
I've been using my FA 28mm more recently; the build quality isn't as nice and my subjective feeling is that it doesn't sharpen up quite as quickly as you stop down but autofocus is a nice feature to have. The F and FA 28mm seem to sell for more like $200+.
I haven't handled an A 35mm enough to have much of an opinion, although they seem be much less common and the build quality is on par with the other A series primes.