Add the outerheight to the top and you have the bottom, relative to the parent element:
var $el = $('#bottom'); //record the elem so you don't crawl the DOM everytime
var bottom = $el.position().top + $el.outerHeight(true); // passing "true" will also include the top and bottom margin
With absolutely positioned elements or when positioning relative to the document, you will need to instead evaluate using offset:
var bottom = $el.offset().top + $el.outerHeight(true);
As pointed out by trnelson this does not work 100% of the time. To use this method for positioned elements, you also must account for offset. For an example see the following code.
var bottom = $el.position().top + $el.offset().top + $el.outerHeight(true);