Textview with totals based on dynamic ALV selections

Hi all, it´s me again, suffering like a mother with my first WDA serious application

I have this ALV with a prices column, with an editable checkbox field, and what my client wants is a field on the screen (I think it must be a Textview) that displays the total sum of all selected rows in the ALV. Of course, this sum must be dynamic, according with the actual selection.

Could anyone tell what should I do?

Thanks a lot, best regards.

Federico Alvarez