Definitely find out if they "Pentax-A", "Pentax-M", or just plain "Pentax" (aka, "K", even though they won't actually say "K" on them). I don't know much about the 35/2.8, but the somewhat newer autofocus 35/2 was very much liked. As for the 28/2.8, the "A" version usually sell for somewhat over $100 US, the "M" version a little. There is no "K" version (although there is are f/2.0 and f/3.5 versions). If you haven't read up on how to use "M" lenses, do that before buying (there are threads about that here and on dpreview) - you might decide you'd rather spend the extra money on "A" if you're averse to using "M" mode on your camera.
Here's a site with lots of useful info on various older lenses:
Stan's Pentax Photography
No price info, but you could always check completed Ebay listings to find out the 35/2.8. Also go the KEH.com and see what they are charging.