I think the term you're looking for is "inflation". It's kind of funny how people get stuck on specific price points for some things. Another good examples is video games - for whatever reason people don't expect to pay much more than $60 for even AAA games, a price point established around a decade or so ago. If they had kept pace with inflation [after a quick google search for inflation calculator], they'd be around $80.
Everything from wages for the people writing the product to the paper it's printed on to shipping costs have risen. I'm surprised it isn't higher.