Depends what you mean by decent. My old unit is 2.1 megapixels and is sufficent resolution for downloading to my computer and 4 X 6 to 8 X 10 prints.
You might be able to pick up an archaic unit cheaply but digital cameras are like all electronics, the price doesn't change but you get more bang for your buck as the technology advances.
They eat batteries so you'll need nickel metal hydride rechargeables. The other item to watch for is your download method. Make sure its convenient for your existing computer. I have a flash card in my camera that fits into a PCMCIA card for my portable.