How did I calculate the power for this source?
There is a small miscalculation. The transfer function for the dependent current source is

$$
0.5\cdot V_y
$$

so if, \$V_y = 10\$, then the current through the source is 5A, multiplied for 10 V (\$V_y\$), is 50 W. The voltage polarity of \$V_y\$ and direction of current \$I_y\$, indicating that the source is absorbing power.