I used the playback function of Flightradar24 for the 18th at 23:00 UTC, and the amount of traffic above 10,000' (filtering by altitude) seemed very normal compared to other days. I'm baffled as to why they flew so low, but I can address your fuel question in some detail.
The difference in fuel consumption is ~693 kg of fuel, and would cost an extra ~$415, which is certainly not disastrous, but can be a good profit on another short-haul flight (volume economics).
The decision to go ahead and fly low, as has been covered, is not just about fuel. There is a myriad of direct and indirect costs associated with cancelling a flight, and its knock-on effects.
How I arrived at the fuel figure:
The distance flown was ~360 NM (nautical miles). The more direct no-weather route takes 307 NM.
I'm limited by 10,000'/29,000' as the upper/lower limits of low/high cruise figures.
Cruising at a weight of 50 tonnes:
At 10,000' each engine burns 1028 kg/h while doing 280 knots true airspeed (indicated airspeed is 242).
At 29,000' each engine burns 1305 kg/h while doing 462 knots true airspeed.
Now to subtract the distance and fuel used in the climb/descent:
To 10,000' takes 11 NM and 336 kg of fuel (at a brake release weight of 52 tonnes).
To 29,000' takes 66 NM and 970 kg of fuel.
From 10,000' takes 23 NM and burns 48 kg.
From 29,000' takes 64 NM and burns 114 kg.
For 10,000', that leaves 326 NM of cruise, which would take 2393 kg (two engines). Adding the climb/descent fuel would be a total of 2777 kg (trip fuel).
For 29,000', that leaves 177 NM of cruise (using the shorter distance), which would take 1000 kg (two engines). Adding the climb/descent fuel would be a total of 2084 kg (trip fuel).
Note: spending the shortest time in cruise (a flight that is mostly a climb followed by a few minutes in cruise) is the right strategy for short-haul flights, see: What should be the minimum time spent in cruise (for e.g. a B737)?