Originally posted by herzzreh Well, I would go for an older manual lens, but finding one that lets me keep the automatic metering (while I don't particularly mind MF, I don't want to mess with stop-down metering) is a pain...
Well, if you don't care about focal length and you don't want stop-down metering, then you can go for an older (and cheaper) M42 lens. There are tons to choose from, including the Industar that I already mentioned. They'll be manual focus, but you can use them in Av mode, just preset the aperture and press the shutter. I think anything between 24-50mm will be generally useful. Takumars and Vivitars in M42 are probably all decent and good value for the price, but there are other options too, if you want to explore a bit.
Leaving M42 aside, you can obviously look for used lenses with an A setting. You can find something cheaper than a new modern lens.
The DA 40 is probably the cheapest prime available for Pentax these days, now that the FA50 price went up. I don't think today there is any cheaper prime lens still in production for Pentax. So that's not bad at all, but if you would be happy with an old lens, then you can probably get half a dozen old ones for that price.