public static inline function commaFormat( num:Float ): String
    var str:String = Std.string(num);
        var str_ar:Array<String> = new Array<String>();
        var pos:Int = 1;
        var i:Int;
        var out:String = "";
        
        if (str.indexOf(".") == -1 ){
            str_ar = str.split("");
        }else{
            out = str.substr(str.indexOf("."));
            str_ar = str.substr(0, str.indexOf(".")).split("");
        }
        
        str_ar.reverse();
        for (i in 0...str_ar.length){
            out = str_ar[i]+out;
            if (pos++ == 3) {
                pos = 1;
                out = ((Math.isNaN(Std.parseFloat(str_ar[0])) && i > 1) || (!Math.isNaN(Std.parseFloat(str_ar[0])) && i >= 1) && i < str_ar.length-1) ? ","+out : out;
            }
        }
        return out;
}

trace(commaFormat(123));
// output: 123

trace(commaFormat(123456));
// output: 123,456

trace(commaFormat(1234567890.12345));
// output: 1,234,567,890.12345

can be written better, however this should be fast enough in most cases.

var i:Int;
var s:StopWatch = new StopWatch();
for(i in 0...10000){
    Utils.commaFormat(123123123123123123123);
}
s.stop();
trace(s);

0.767 ms