Sites charge what they can get, or what they hope to get. This is not just a music publishing thing. Most scientific journals cost a couple of hundred dollars a year for a subscription, something like two or three hundred articles, say a buck or two a piece in that format. You can typically order the article as a download from the publisher for 25 to 30 dollars each. And this is a system in which the authors get no money and often have to pay editorial and in some cases publication costs, the peer reviewers work as volunteers, and the editors get a pittance. If you need the data in a hurry and have a budget for a literature search you will probably find it cost effective compared to going to the stacks in the library.
Song lyrics are freely available because in many cases they are pirated, i. e. the site does not pay the author or copyright holder any kind of royalty at all for hosting the post.
http://www.grammy.org/files/pages/sam_guthrie.pdf Publishers are probably more aggressive at enforcing their rights for the sheet music, and the average kid who has nothing to lose by posting his best guess at the lyrics is not so likely to be able to turn out a good transcription of an arrangement.