As of jQuery 1.6, you can do this most easily by simply adding or subtracting from the current value. For example, to add 150px:
$(this).css("left", "+=150")
http://api.jquery.com/css/#css-properties
As of jQuery 1.6, .css() accepts relative values similar to
.animate(). Relative values are a string starting with += or -= to
increment or decrement the current value. For example, if an element’s
padding-left was 10px, .css( “padding-left”, “+=15” ) would result in
a total padding-left of 25px.