span is not a block level element, so I don't think you can position it relative to another element.
Make it a div with style="position:absolute;right:0;bottom:0;text-align:right'
Make sure that the containing div has style="position:relative;"
to contain the absolute object.
JSfiddle: http://jsfiddle.net/gHZqs/
Edit
You can use the span tag instead of the div, as the others have said. However span tags are for inline content. If you take the content out of the flow with absolute positioning then you should use a div IMO. If you're working within a framework or something and you need to use the span tag then I wouldn't lose any sleep over it. Here's a link on a similar question:
SPAN vs DIV (inline-block)