I consider the following to be a terrific lens kit for the cameras you own. SMC Pentax M 35mm f3.5 (a beautiful wide angle), SMC Pentax M 50mm f1.7 (the perfect fast normal for low light), SMC Pentax M 135mm f/3.5 (a marvelous portrait lens), and the SMC Pentax M 200mm f/4 (a light, but very good, telephoto). If you keep your eyes open you can usually pick up each of these at or below your price point. They are very good lenses and are very sharp and effective. Because they are not the "speediest" in the land they don't usually get a lot of attention so can be picked up quite reasonably. I use these lenses a lot with my K1000 and they never cease to exceed my expectations. I think it will be a long, long time before I actually outgrow these lenses, if ever.

Later, if you want, you can go looking for a nice 50mm Macro though these will typically go for a little bit higher than what you want to pay right now. The only other one I might consider would be the SMC Pentax M 85mm f/2 (a very capable short telephoto.)