When creating a +1 Longbow, Composite (Str +1), the correct price shows up when you click on the 'Edit this weapon' button, but it adds 100 gp to the item when you hover your mouse over the '?' button.
(+1) + (Masterwork) + (Longbow, Composite) + (Str +1) = 2000 + 300 + 100 + 100 = 2500 gp. The '?' has 2600 gp listed.
For each increase in the Str rating for the Longbow, Composite, the price of the item when you hold your mouse over the '?' button incorrect raises the cost by 100 gp, no matter if its regular, masterwork, or magical.
I also believe this value throws off the 'Wealth' and 'Gear Value' number stated at the top.