I've got two battery grips, one for the K10D and K20D, and another for the K5. Both eBay Chinese ones, both relatively cheap. The earlier one has slightly flaky controls, but the second one for the K5 is perfect, although, of course, not weatherproof like the genuine article. I've had them both for quite some time now.
Ignoring the control wheel issue on the early one, they both allow me to have double the battery life and in the case of the later one, use AA batteries if need be. I don't have them on the cameras all of the time, just when I think I might be taking a lot of vertical format pictures or if I expect to use the second battery.
So I guess what I am saying is there is a lot of choice out there, and it is worth looking at the offers on eBay if you are somewhat parsimonious.