I'm sure everyone would love to see that in their post box. "We decided to add some new bushes in town, that'll be a 10% increase in council tax please."
This is usually meant to retain enough water to prevent building lots of grey infrastructure. The Thames Tideway tunnel to lower the amount of sewage outflow in London is 4.3B pounds. In that context, building a lot of mini-retention ponds for tens of thousands of pounds is a lot cheaper. and this isn't the only strategy; a lot of modern parks, for example, are built with facilities like courts for sports below the ground level of the park, so that during flood times they turn into bathtub holding ponds as well.
This isn't a universal problem, but it means there's a pretty big benefit to reducing runoff, and the city has been pursuing it aggressively for the last 20 years or so, to good effect.
(The runoff also leads to localized road flooding, which is again both dangerous to road traffic and expensive to mitigate.)
Not only that, green space helps reduce the heat island effect and manage/capture storm run off. Both saving a lot of money and resources.
It might net neutral or net benefit. Now the lack of maintenance and underinvestment is visible, but you're taking strain off the (mostly invisible and no doubt underinvested in) storm water system by swapping impermeable surfaces for permeable ones. (This is a good way to save money on storm water systems in general).
That said, my cynical side would still somehow expect my local council to screw it up anyway.
I do not know what the impact on maintenance costs of greening public spaces is though - hard surfaces need less frequent maintenance, but it is more expensive when needed.