Inventory Cost Dropping in price after each sale?

Help required please. I have sold 9 of the same item during July. My Cost price is $45.with no more purchases during the Month.
My 1st item sold was costed at $45.00 * 2 sold on 1 order.
2nd $40.40
3rd $38.33 * 2 sold on 1 order
4th $34.40.
And so on etc dropping in price This is happening on all my items.
Looking forward to your replies to correct my situation.

This was a bug. Fixed in the latest version 14.8.13.

Lubos & Staff, Thank You I could not ask for better service.

Cost price of my products changes on daily basis. When I change the cost price it change the amount of inventory in hand, inventory sale expense, net profit.

It is possible that when i change the cost price no changes made effect on previous invoices etc. After changing cost price it may effect onward invoices after changing.

Thanks & Regards,

Where do you change cost price? On transactions which are purchasing inventory items?