Something like this should work as the Mouse Up script of the button:
// Amount to add
var amount = 2.72;
// Field to add amount to
var f = getField("cost");
// Add the amount to the field, rounded to the nearest 100th
f.value = util.printf("%.2f", +f.value + amount);
This assumes the field you want to add the amount to is named "cost" and that has a numeric format.