Formula referencing conditional formatting

I have a data set that is conditionally formatted to highlight the last row of similair data. There can be anywhere from 1000 to 4000 rows with several columns and it constantly changes so I need something dynamic.

Here is what my data looks like:

Column A Column B Column C

New York $400 4.1
New York $500 5.0 (it highlights this one in conditional formatting)
L.A. $600 6.0
L.A. $750 3.2
L.A. $800 1.2 (it highlights this one in conditional formatting etc..)

What I need is some way to get a weighted average of the two numbers above. So for New York i need "((400*4.1)+(500*5))/sum(500+400)" and the same for L.A. I can do that with a "sumproduct" by hand for a small data set, but when it gets really large and starts to change every other month I need a macro or a formula or something that can look at the name in column A or count up from the conditional formatting and be able to give me that sumproduct dynamically.

I think I've exhausted my knowledge of excel so any help would be appreciated (if my idea is even possible to construct). Thanks all!