Subtotal Each Cell In A Column?

I have a sheet with about 3000 lines that looks something like this:

A...............B..................C
1234........$42.................Subtotal of all values in group "1234"
1839........$58.................Subtotal of all values in group "1839"
1837........$15.................etc
1234........$11.................etc
1592........$19.................etc

The numbers in column A refer to a specific "group" number - anything in this group I want to subtotal into column C.

I'm not sure how to do this though. I've looked at the =SUBTOTAL function, but I'm not sure exactly what to do to make what I'm trying to do work.

I wrote a For Each loop to go through each cell and add values, but that took like 4 hours to run - so.. that's not the way to go lol.