There are quite a few ways to spend $150 on macro. I might be doomed to try them all.
What separates the different methods are magnification, image quality, distance from subject, camera and lens automation, and ease of use. If you can get specific about your subjects, you can eliminate a few options. The DA 18-55mm kit lens does 1:3 magnification, so you need 1:2 or better.
Lee is right about the Phoenix, also sold as Vivitar, Cosina and even Pentax, in manual focus and autofocus versions. If you really want autofocus, it's the only dedicated macro lens that will fit your budget. Another macro lens that might fit within your budget is the Tamron Adaptall-2 90mm f2.5 Macro. The Adaptall-2 system means you also have to track down an adapter for K-mount, and if you need more magnification than 1:2, a teleconverter or extension tube. With some deal-hunting you can do this and have a completely manual lens. It has much higher build quality than the Phoenix, maybe not that much better image quality. A third possibility is the Pentax-M 100mm f4 Macro, also 1:2 magnification and manual operation, but no adapters required.
The Raynox 150 and 250 diopters are good options too. They attach to an existing lens, so you can have the featues of that lens available. They'll leave you with some money leftover, and some combinations can get to or exceed 1:1 magnification.
Another possibility just over your budget is the Tamron 70-300mm f4-5.6 zoom, that can do 1:2 magnification. Here is a comparison between that lens and the Tamron Adaptall-2 90mm macro that I just did:
Centers Corners
Note that the zoom was at 300mm and 37 inches away, while the 90mm lens was 15 inches away. The images are about the same size because the two lenses have the same magnification ratio. The focal length affects the distance from the subject. The zoom is not as good a lens but useful for telephoto too.
That's only scratching the surface of available options.