suggest you use Firebug to get an idea of the rendered row column ids.
I tried with Firebug but still i couldn't fix that,
i see the span class id as t1:0:it4 (0- row index) and inside the span i could see the input class id as t1:0:it4::content and name as t1:0:it4.
I tried with span class id & input class id but still getting object or undefined error.
// var comp =document.getElementById("t1:0:it4").innerHTML
I got one solution were i found in a forum,
var comp = document.getElementById("t1:"+rows+":it4::content").value;
var result =comp * 5;
document.getElementById("t1:"+rows+":it4::content").value = result;
The changed value is effected in table if the column is editable and if it is readonly this change is not reflecting in screen.