These are the 'back of the envelope' calculations I came up with to see if it was cheaper to run electric then diesel.
Based on a load of assumptions and guess work I worked out that over a 2000 cycle life of my LiFePo4 batteries with a 70 mile range they would have driven 2000x70=140,000miles.
Assuming 40x200ah cells at approximately £1/ah that gives £8,000 for the cost of the battery pack.
The pack would have 25.6kWh and at 80% DoD it would be 20.48kWh. To recharge that amount per 70 mile trip would cost 20.48x0.15=£3.07 at 15p per kWh. Over the 2000 cycles that would be £6,140.
Total cost £13,140 giving 9.4 pence per mile.
Over 140,000 miles in my diesel car at 50mpg I would use 2800 gallons of fuel. Assume £5/gallon that would be £14,000 and 10 pence per mile.
Granted that the ICE will have higher maintenance costs and that both fuel supplies will increase in cost over the 140,000 miles it can be seen that it is not always a straight forward case of a financial saving in running an EV. If the EV had a complicated maintenance shedule with repairs to controller, charger and battery management, replacement cells, etc then the maintenance saving could be even less or non existent depending on how much labour on either vehicle could be done as DIY. The there is also the cost of conversion labour and parts on the EV donor.
At 140,000 miles the EV could be looking at a replacement battery pack. The diesel ICE car could do the same distance again, or more, on the same engine. Not a direct comparison as the EV's motor could outlast the ICE easily.