There are short term costs: the immediate cost of foot trimming and treating the lame cow, and long-term costs: the increased chance of her being culled early or impact on calving interval.

Many papers have tried to estimate the cost of lameness per lesion such as Willshire and Bell (2009). Most have been estimated using known losses to milk yield, fertility and longevity added to the costs of treatment. A simple spreadsheet can be used to calculate the costs taking into account some reduced inputs. Here is a calculator which does this for you: CALCULATOR 

Archer et al. (2010) found persistent mobility score 3 cows lose approximately 1000 litre of milk per 305d lactation. Whilst persistent mobility score 2 cows, lose approximately 500 litres of milk per 305d lactation.  

Figure 8: Research papers estimating losses per lesion 

AHDB Lameness Cost Calculator is a useful tool to help producers calculate the impact of lameness on their farm business.