my_number = "12587654325";
// array in sections of 3 to separate with commas
trace(my_number);
var dollar_array:Array = new Array();
var start:Number;
var end:Number = my_number.length;
trace(my_number.length);
while (end > 0) {
trace(end);
start = Math.max(end - 3, 0);
dollar_array.unshift(my_number.slice(start, end));
end = start;
}

// assign a comma delimited value from dollar_array
my_number = dollar_array.join(",");
trace(my_number);