I think you'll find that single sided boards cost the same as double sided. The exact price depends, of course, on the size and quantity of your boards. And the cheapest manufacturer for one size and quantity might not be the cheapest at another size and quantity.
Check out
PCBShopper.com. It's a price comparison site for PCBs. You enter your board's specs and it shows you prices from up to 24 different manufacturers, including some of the best known companies in Asia, North America, and Europe. You can also read Amazon-style reviews of the manufacturers.
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- Bob Alexander
PCBShopper.com