Check out this.....
function CalculateSum()
{
var grid = document.getElementById('<%=gvAttribute.ClientID %>');
var gridinput = grid.getElementsByTagName('input');
var sum = 0*1;
for(i = 0; i < gridinput.length; i++)
{
var value1 = gridinput[i].value;
sum = parseFloat(sum) + parseFloat(value1);
}
return parseFloat(sum);
}