Vlookup and Formula needed to calculate total cost by monthly basis

Vlookup needed to generate monthly cost, but date range is the determining factor.

1. Volumes are presented on a daily basis (sheet 2)
2. If date range is for one month (ex. January) then daily volume needs to be multiplied by 31 days and the daily cost = monthly cost
3. I would like this to be one vlookup/column, if possible.

Thank you.Vlookup.xlsx

Hi,

Quick question: I'm looking at your spreadsheet now, and if I do the sums manually (without a formula, as I haven't worked one out yet :P) I actually get a monthly invoice amount of $13,260 for item A, whereas you have suggested it should be $13,169.18. So I'll explain my methodology, and can you please tell me what I am doing wrong in order for me to be able to help you out?

I manually worked out the number of days for each month listed on Sheet2, then multiplied each of those values by 0.4 (Total Daily Cost of item A on Sheet1), and then multiplied each value again by the volume (listed in column B on Sheet2 for item A). I then averaged the 12 values to get a monthly cost figure and my resulting answer was $13,260 as mentioned above.

So can you please explain to me what I need to do differently, and it may also help to explain to me your methodology in getting an answer of $13,169.18.

Thanks

Cancel that last post, I just realised it was a rounding error, as "0.4" is actually "0.397260273972603".

My bad