Around 20-40 kwh or so for a single journey assuming a maximum capacity of 100-150kwh and assuming it would be rare to use more than 20-30% of it on a single journey or fly the maximum range. At 5-10 cents per kwh (i.e. 20-40kwh/$) we're not even talking some fancy barista doing their thing. Grid prices can be higher of course but I'm assuming a bulk consumer of energy would invest in turning this in a fixed cost by e.g. installing solar panels & batteries to lower the cost.

By the time these things start flying at scale, the coffee will be way more expensive than the energy cost. That's why most of these companies are investing in autonomous flight as well because short term that is going to be the cost bottleneck (that and training the army of pilots that they'd need).