Tbh I feel like that's probably an underestimate, but otoh that's probably a fraction of the cost of maintaining the trees themselves, so I'm not sure it's worth fretting over.
And the whole point of having a government of London is because it's a huge city. If it was just a large car park it wouldn't need a government. The city is for people, and people both like trees and benefit from many properties of these trees such as their ability to absorb rainfall and provide shade as well as positive contribution to air quality (well, unless you're allergic to tree pollen) so it makes sense for the city government to look after them.
This is therefore a reasonable expenditure of some government resources. Just as when they paid "Royal de Luxe" to perform The Sultan's Elephant over several days. The Sultan's Elephant is street theatre, an enormous mechanical elephant, a giant little girl, and some other apparatus turns up, the puppeteers from Royal de Luxe operate these puppets to tell a story for the entertainment of just... everybody who happens to be there, it's on public streets, it's not ticketed, it's not in some arena or screened off area, it's just in your city where you live and/or work. Perhaps you came specifically to this part of the city to see the giant little girl, asleep so peacefully, or perhaps you were surprised to find your journey interrupted by an enormous elephant walking down the street when you went to buy lunch, it doesn't matter, this sort of street art is a reasonable investment.
