The company that makes the cheapest boards for one size is not going to make the cheapest boards for all sizes. There is no such thing as "the cheapest" PCB manufacturer.
Once you've designed your board, you should go to PCBShopper.com.PCBShopper.com. It's a price-comparison site for PCB manufacturing. You enter your board's size, number of layers, soldermask color, silkscreen options, quantity, the country to ship to, and how quickly you want them to arrive, and PCBShopper gives you prices from over 20 different manufacturers.