proc format_usd {num {sep ,}} {
    # Find the whole number and decimal (if any)
    set whole [expr int($num)]
    set decimal [expr $num - int($num)]

    #Basically convert decimal to a string
    set decimal [format %0.2f $decimal]

    # If number happens to be a negative, shift over the range positions
    # when we pick up the decimal string part we want to keep
    if { $decimal <=0 } {
        set decimal [string range $decimal 2 4]
    } else {
        set decimal [string range $decimal 1 3]
    }

    # If $decimal is zero, then assign the default value of .00
    # and glue the formatted $decimal to the whole number ($whole)
    if { $decimal == 0} {
        set num $whole.00
    } else {
        set num $whole$decimal
    }

    # Take given number and insert commas every 3 positions
    while {[regsub {^([-+]?\d+)(\d\d\d)} $num "\\1$sep\\2" num]} {}

    # Were done; give the result back
    return $num
}