Generating average monthly values based on weekly values

Hi everybody,

I've run into a bit of trouble with some excel work so I thought id post here to see if u would be able to help me . Basically what I am trying to do atm is generate a formula that takes all weekly prices, and in turn generates monthly prices off them automatically.

The problem I run into is due to the irregularity in number of weeks in a month, I cant create a formula that averages at a regular interval. Rather, it somehow has to group averages based on the number of a the month of each week. I've included a picture of more or less what i am working with (unfortunately i cant disclose actual price values)

Any help would be greatly appreciated

rgrds,
Marcus