We expect councils to charge between £100-£200 to residents to sponsor a tree, this will depend on the financial circumstances of each council. Some councils may choose to ask for a smaller contribution in less affluent neighbourhoods.
The background to this is that street trees are very expensive and can cost upwards of £500 each when planting and watering costs are included. This can be higher still if a new tree pit needs to be created.
If you are asked to do the watering this will save up to £150-£200 of these costs, which are the normal contractor fees to water each tree over its first 3 years.