About how much does a pet hamster cost to care for monthly?

I'm thinking of purchasing a hamster soon for my son's ever nearing birthday. I would just like for someone who has one to give me a rough estimate on about how much they cost per month. Such as food and things go. And also do you think I should get two? I mean I'd hate for the little fella to be alone all the time with none of his kind to keep him company. Or do they not mind solitude? I obviously never had one as a kid and need a quick lesson on these basic hamster facts and details haha. Thank you very much