Why not 3-digit groups starting from the right? As in... almost everywhere.
12,345,678 things.
$12,345,678
It seems to me that the only good reason to deviate from this for non-countable things that don't have an established grouping would be any research that shows groups of 4 starting from the left is better in some way. Off the top of my head, since it's natural to pause between groups when speaking, in theory more digits-per-group = fewer groups = fewer pauses = more efficient.
In any case, number grouping is something interesting to think about. It's definitely annoying to search the back of a TV and find that the serial number is 16 digits crammed together in a 2 pt. font!