Need help summarizing monthly avearges from changing references

I would like to get some help on a task calculating averages from a complex set of data in Excel.

I need to display the average price per brand for each month of the past year from an Excel file that contains brand/product/price data per month, on separate sheets by months. The problem is that the product references change over time, as new products are added and old ones are removed (the brands are not changed) - so a brand's products can vary in the list month by month. Thus, I would need a solution that for any given 12-month period can give the monthly average price per brand, based on only products that are referenced in every month within the period.

The data is currently organized the following way, each month is on a separate sheet for the past 2 years:

Aug 2014

Brand	Product	Price
A	A1	10
A	A2	15
B	B1	5
C	C1	10

This is the output what I would like to get:

Brand	Sep 2013	Oct 2013	...	Aug 2014
A	10	10	10	12.5
B	4	4.5	5	5
C	10	10	10	10

Thanks for any ideas!