From the website of manufacturer, Tesla Motors we're taking the specification.

What we have there when it comes to a Battery usage:

- it takes 3.5h to fully charge battery at 240 Volts and 70 Amps. What does this mean?

To calculate the cost we need to know how much power do we need to fully recharge. 1W [Watt] = 1V [Volt] * 1A [Amper]. So to fully charge the Tesla Roadster battery we need 16800W = 16,8kW.

On a fully charged battery we can make 245 miles, so approximately 360 km.

On this website we check the costs of energy in different countries of Europe.

Few examples:

Electricity prices:

• United Kingdom cost of 1kW is 0.138 Euro.
• In Netherlands cost of 1kW is 0.241 Euro.
• Poland cost of 1kW is 0.14 Euro.

Fuel prices:

• United Kingdom cost of one litre is 1.41 Euro.
• Netherlands cost of one litre is 1.53 Euro. 
• In Poland cost of one litre is 1.17 Euro.

To make 400 km, in two scenarios, one using electric car, second using fuel powered car in three different countries we will need to spent:

• United Kingdom, to make 400km on electric we need to spent 2,32 Euro, on a fuel 23,15 Euro.
• Netherlands, electric car 4.45 Euro, on a fuel 22.95 Euro.
• In Poland, for electricity powered trip 2.6 Euro, on a fuel 17,55 Euro.

As we can see this can be an amazing investment for companies and normal people as well, but keep in mind there are many other factors to incorporate to calculate the profitability (service costs are the most obvious, and I'm very interested in them, if you have any data about it please let me know).