I think,
insertting commas every 3 numbers is a method, but not a good one.
format(price, ',d'), or something like that is clever but not a suitable one.
It is the feature of finance respect of every 3 numbers a comma.
format(price, ',d') solves it by chance.

In my opinion, I used the recursion method. If you like it, you can check my code.
Enjoy...