Determining Cost Period based upon number of Tags in a 6 Month Range

See the attached spreadsheet. Book1.xlsx. I am using Excel 2010 and have data in 5 columns Tag, Part, Date, Qty, and Cost. In a separate column I need to Compute a bill based upon the following logic:

If less than 3 tags have been filed for a particular part number during any 6 month time period then now additional bill is needed.

An additional cost needs to be calculated for any period of time that tags were filed for a particular part at a rate of 3 or more tags every 6 months. Once this cost period has been established, the additional cost calculation is as follows:

The additional cost will be the lesser of $1000 or Qty*Cost*25% over each separate cost period.

I am at a loss as to how to start making this calculation and any help would be awesome.

After looking at your method Richard, it doesn't appear that I was clear in what I was asking. I will focus only on the first part of my request counting the number of tags in a 6 month window (I have approximated 6 months to be 180 days).

I need to count how many tags were issued on a single part number looking back in a six month window from the date listed on a Tag. For example Tag B87343 is dated 30-Oct-02, looking back to 3-May-02 there is only 1 tag for the part number 17AB123XX so I want a 1 to be returned. For Tag W04069 dated 4-Nov-04, looking back to 8-May-04 there are 5 Tags that were issued for part number 12AB123XX. I have included a spreadsheet with some manually entered output in column H. Book1(2).xlsx.

Thanks Richard that worked great.