var grid = jQuery('#list1');

var sum,a, count=0;
var ids = grid.jqGrid('getDataIDs');

for (i = 0; i < ids.length; i++)
{
    a = grid.jqGrid('getCell', ids[i], 'amount')
    if (a != 0)
    {
        sum += a;
        count++
    }
}

var avg = Math.round(sum/count,2);
grid.jqGrid('footerData','set', {id: 'Avg: ' + avg});