I agree with bartosimpsonio.
If you get highly targeted people who are ready to buy (keyword: weight loss pills for 30 years old women under $50 in USA), then the conversion rate would be higher. Assuming you have a great website, a specific market that you are targeting, and more variables.
It depends on too many variables. Your target audience, the product price, the kind of product, your ad text, ad quality and so on. Can't just say. PPC can vary wildly from fractions of a percent to over 15 or 20%.
I will use banner advertising, and the traffic will be targeted. The price will be $3.99 and I'm working on my sales page since 6 months ago. It has a video, a good title and will have good bullet points...
you've been working on a landing page for 6 months and you have no idea of the ROI or PPC or anything? I wonder what you did during these 6 months honestly.