Most shops will charge about an hour to check codes AND diagnose the problem, checking codes ONLY (no further diagnosis) would be less.
Now, realize that those codes in themselves don't always tell the tech exactly what's wrong and the info (codes) read may not mean the same to all techs, as strange as that might sound. A fuel trim code for example doesn't lead the tech to a specific part so sometimes that info is not always helpfull to an inexperienced person who could then still guess incorrectly, usually at your expence. So, what I'm saying is that if you're going to pay someone, it's adviseable to be sure that that person knows the product, not just a guy who claims to work on Volvos occasionally.