Well, the data we do have is that the flight was just under 30 minutes and very likely under 100 miles.

From the Wikipedia on airline fuel efficiency:

The worst-performing flights are short trips of from 500 to 1500 kilometers because the fuel used for takeoff is relatively large compared to the amount expended in the cruise segment, and because less fuel-efficient regional jets are typically used on shorter flights.

In the example values table, the most efficient plane for a 560km trip burns 0.92 kg of fuel per km, so doing some rough math and assuming the electric plane travelled 100 miles, that would be roughly 148kg of fuel, or 50 gallons (190L).

At current jet fuel prices ( $3.76/gallon ) that's about $188 US in jet fuel as a rough estimate. It's unclear if the test flight went up to full altitude or if the plane was at full weight, so a fair comparison might have used even half as much jet fuel.

Edit: From some of the other comments, it seems like they might have only considered flight time as cruising time, not takeoff and landing, so my numbers will be quite far off if that's the case. My gut feeling is that this is probably the case, because this seems like too big a difference otherwise.