The easy rule of thumb is that each target after the first is worth 50% of the first target. This doesn't keep multiplying; 1 for 1 target, 1.5 for 2, 2 for 3, 2.5 for 4, etc.
Based on that, Command scales roughly like Cure Wounds.
At +5 wisdom, the healing power-per-(0.5 + slot level/2), assuming 4 targets on an AOE heal, is
HW: 7.5, 6.7, 6.3, 6, 5.8, 5.7
CW: 9.5, 9.3, 9.3, 9.3, 9.2, 9.1
MHW: -, -, 9.4, 10.0, 10.4, 10.7
MCW: -, -, -, -, 15.4, 16.4
To improve this model, I'd assign a subsitution value to your action (maybe like 5+character level) and your bonus action, and subtract that from the value of the spell before pulling out the slot value.