DfT have a calculation which assesses the costs of fatal, serious and non-serious casualties. We have linked the 2015 casualty stats for each local authority to this formula so that you can see the cost for your area.
Public Health England also make an assessment of the costs of physical inactivity. Based on your authority's population we can show this for your area.
We have also shown an approximate one-off cost of implementing 20mph limits across the authority based on the typical £3 per head of population. This is then shown as a percentage of the total annual casualty and physical inactivity costs.
Whilst it is accepted that this is an approximation and may vary according the distribution of population it does show that put against the annual casualties and physical inactivity costs, the one-off cost of 20mph limits is minimal and typically less than 1%.
These calculations are available in a spreadsheet which you can download here.
Once downloaded then after "allowing editing" simply select the blue cell and then use the drop-down box to select your authority.