Specify a static id to your grid, for example, igitems.

Now create a dynamic action on the amount column of IG for change event. Create a true action as Set Value. Set the type as PL/SQL expression. Specify the PL/SQL expression as :AMOUNT. Submit item AMOUNT. Set affected items Px_AMOUNT. Then create another true action to execute JavaScript code and add the following code in it:

var model = apex.region("igitems").widget().interactiveGrid("getViews", "grid").model;
var n_amt, n_totamt = 0;
col_amt = model.getFieldKey("AMOUNT");
model.forEach(function(igrow) {
    n_amt = parseInt(igrow[col_amt], 10);
    if (!isNaN(n_amt)) {
        n_totamt += n_amt;
    }
});
apex.item("Px_AMOUNT").setValue(n_totamt);

Suppose you have an item Px_AMOUNT on your page. The above program will calculate the sum of the amount column and will set the value of the Px_AMOUNT item.