Jan.1

Question

Beginning Inventory..300 units @ $3.00
Mar.5
Purchase..........… 900 units @ $4.00
Sept.3
Purchase...........… 1,000 units @ $5.00
Nov. 4
Purchase....… 500 units @ $7.00

During 2011, 1,000 units are sold at $10.00 each. Assume that the Periodic Invetory System is used.

I am asked to compute the Cost of goods available for sale on December 31, 2011.
My logic is:
300 x 3 = 900
900 x 4 = 3600
1000 x 5 = 5000
500 x 7 = 3500

900 + 3600 + 5000 + 3500 = 13000
however this is where i'm confused. Would the Ending Inventory and the Cost of Goods available for sale on december 31 be the same? So:

Since during the year they said they sold 1000 units at $10.00 each meaning 1000 x 10 = 10,000.
Would the Cost of Goods Available for Sale on December 31 be 3000 (which is also what ending inventory would be)? Or would the cost of goods available for sale be the 13,000?