So you want to buy some electrons ?
Let's take a quick look at the picture on the first page of our Learning Zone. From this you can see that the number of electrons are conserved. So unlike traditional chemical reagents, you don't get to add electrons, you just move them from one molecule to another, as a result of the electrical circuit. In other words, you are paying for the flow of electrons through your reaction mixture.
So thats the question we are going to answer - how much does it cost to flow electrons through the circuit ? The current tells us about the flow rate of electrons, whilst the potential energy across the electrochemical cell tells us about how much energy the electrons require to flow through the circuit (the dominant resistance is the electrochemical cell - losses in wires etc will be tiny). The resistance of the reaction mixture (influenced by many factors explored here) links the voltage and current. Put this all together with the cost of electricity and you can get an idea of cost.
Just a note on units: the cost of electrons will be given in whatever units the price is per kWh. If your price is cents per kWh, then the cost will be in cents, if you're price is in tea bricks or Rai stones per kWh, then the cost will be reported in tea bricks or Rai stones, respectively. You get the idea!