For clickbank your best option is to find easy to rank, low competition long tail keywords for your evergreen niche product. You choose a product that has a big money following like health or make money online, and you target long tail keywords so that the traffic will still be interested, but your competition is not bearing down on it like a nuclear bomb. 20 budget is nothing, save your money. If you have no money to promote clickbank products, move on to something else that will give you money so you can promote clickbank products effectively.