Why on Earth you assume that someone starting to use jQuery will come from PHP background, and more over, why should that person think that $FOO in PHP is cheap

Why on Earth you assume that someone starting to use jQuery will come from PHP background, and more over, why should that person think that $FOO in PHP is cheap? I guess anyone thinking in terms of performance cost will be smart enough to understand what $() does in jQuery, especially when advice to cache selectors is on every wall.