For a true wide angle, you want 24mm at the very longest. 28mm is getting into short-normal territory.
Sounds like the lens you really want is the Pentax M 20/4 or DA 21/3.2 Limited, both of which are tiny but a bit more than you want to spend.
If you want AF in that price range the only option is the Sigma 24/2.8 Super Wide II. It also came in an MF version with KA-mount so you can avoid stop-down metering.
Here are some other MF options. The ones marked with a * can be had in KA-mount, but some had regular K-mount versions as well.
Tokina 17mm f/3.5
Cosina 20mm f/3.8*
Kiron/Vivitar 24mm f/2*
Tamron 24mm f/2.5*
Pentax K 24mm f/2.8
Cosina/Vivitar 24mm f/2.8*
Pentax K 24mm f/3.5
The Tamron came in Adaptall and Adaptall-2 versions...you can get a KA-mount Adaptall-2 ring.