The highest I've seen was 20%, the lowest I've seen was about 0.01%. Partially depends on your market, partially depends on your traffic volume (so you can iterate on tests) and focus. Really you should be asking about the whole funnel and looking at NPV vs CPA or ROI or payback period, depending on your financial and funding picture. I consult on this type of stuff for a living while I'm bootstrapping my startup. I'd try sell to you but I already have one of your competitors as a client so I don't think that would be a good idea. :)