In my understanding, the "power per sample" does not have meaning. I think that you are interested in the "energy per sample" of the converter. It is the energy consummed for obtaining a sample, and is simply the product of power consumption by the sampling period, i.e.:
Es [Joule/sample] = P [Watts] * Ts [seconds/sample] = P [Watts] / Fs [samples/seconds]
Less is better, but other parameters (like accuracy) are not taken into account.
In ordet to compare converters with different accuracies, the effective nomber of bits should be taken into account [bits/sample] giving "energy per bit" [Joule/bit].
Regards
Z