Hi
Unfortunately, it's not straightforward to use your Canon FD lens on a Pentax body, as the flange focal distance (the distance between lens and film / sensor) is less for Canon than it is for Pentax - and there's no way of mounting the Canon lens any closer to the sensor than the body will allow. There may be an adapter available, I'm not sure, but if there is it would have to incorporate some sort of lens - and that would potentially affect image quality from the lens. I think the adaptors you've seen for 20 bucks or so are probably designed for mounting Pentax lenses on a Canon body, which is a much simpler prospect.
There's been quite a bit of discussion here on this subject before. You can read one of the articles
here. Hope this helps.