I would approach this as a strategic invesment decision as follows:
The maximum price I would pay for the bridge = Incremental yearly profit generated by the bridge x the break even period you have as a target (in years)
I would disaggregate the profits into it's drivers:
- revenue would be mapped to the money you generate from the toll on the bridge, which can be mapped to the number of cars crossing the bridge each day.
- costs woule be mapped to maintenance, upkeeep and repairs etc
I would size the market to generate revenue (assuming we know the toll price) and ask for data on costs. Given the calculated profit generation - you multiply by the target break even period in years and this is the maximum price you should pay to meet your target (e.g. if you want to break even on the investment in 5 years and the bridge will generate $50M per year in profits, the maximum price you should then pay is $250M).
Hope this helps! :)