I believe the easiest way to do that is to use the built-in ASP.NET AJAX Sys.UI.DomElement.getLocation() client-side method. Example:
var element = e.target; (the HTML element where click is registered)
var j = Sys.UI.DomElement.getLocation(element);
var x = j.x;
var y = j.y;
getLocation() is officially documented by Microsoft here:
http://www.asp.net/ajax/documentation/live/clientreference/Sys.UI/DomElementClass/SysUIDomElementGetLocationMethod.aspx