Park/Clarke Transform and Power
Always, the product of \$I_{DC}\$ and \$V_{DC}\$ gives you the power. The FOC needs to measure stator currents, not the DC current, which isn't clear from your description if you really measure DC current, voltage.
First, from stator currents ia,ib,ic (or ia,ib for symetric load as AC motor is) you transform into  \$\alpha\beta\$ coordinate system and then into dq coordinate system. The  \$i_q\$ is proportional to the output torque, hence the elecrical power can be computed with the formula  \$P=M\Omega = k_ii_q\Omega\$, where \$\Omega\$ is the rotor speed \$[\dfrac{rad}{s}]\$  

In FOC the voltage is set with PI controller, so that inverter outputs the setpoint  \$i_q,i_d\$ and there is no voltage feedback (unless you have BEMF sensing for sensoroless control). Since you mention about FOC, then you have DQ currents and speed from encoder and this is enough to calculate the electromagnetic output power.