Try marketing the product using some free methods to test if you're going to make sales on it before you invest in ppc on it. Also, while your marketing via free methods, watch the ads of other people marketing the same product and see if in a few weeks they drop off. If they do, the product is probably not going to covert well. If the ads stay up, look at their ad, modify it only slightly, and use it.
Some free marketing methods that you can use are usfreeads, forum signatures, and article writing.