Cheapest option is to download a free landing page design and code it: https://www.sketchappsources.com/

A better option is to purchase something off of https://themeforest.net/category/site-templates

its usually pretty cheap (sub $100) and the quality is pretty high.

Learning to design your own is harder. I'd say start with something from the resources above and iterate as you optimize for specific goals. As you spend more time optimizing your product you'll see design patterns being used by your competitors and learn along the way.