Costs are generally reasonable. Costa Rica has the cheapest electricity rate in Central America (mostly generated by local hydro- and geothermal plants) and by far the biggest connection to the power grid in all of Central America. The voltage used is 110V AC at 60Hz, the same as in the US.
Bills can be paid in banks and supermarkets, but if you don’t pay on time, your service will be cut off, usually within two to three weeks. Although you may be able to negotiate this a little, once the service is cut off you will have to go to the main San Jose or reginal office to pay a fee to get reconnected.