I charge exclusively on public chargers, because I have no way of charging at home. By my maths, I have to be paying about 65p/kWh for petrol to be cheaper to use(assuming 150p per litre, and 40mpg average efficiency). There is a charger near me that costs 75p per kWh, but even GridServe motorway chargers are only 65p/kWh for the lower power units, and 66p/kWh for the 350W chargers. I would say the average rapid charge price in my area is around 55p/kWh (Mix of PodPoint and BP Pulse) for rapid chargers, and about 30p/kWh for slow chargers.
The economics of me using exclusively using public chargers are that I still save money when compared to petrol. Although personally, I would be happy to pay a bit more for the electricity as compared to petrol because of the lower carbon impact.
(For reference, I have a BMW i3, which is currently sitting at 4.0 miles per kWh efficiency. I expect that to go up to 4.2-4.3 in the summer, which would nudge the break even point to more like 72p/kWh)