New members of this site who post at 4am (UK time) are often based in the USA. (This website is UK-based). If that applies to you it will be necessary for you to work out your own costs (because people around here won't know the price of energy in the USA).
However a quick bit of googling suggests that typical tower heaters (such as this one:
http://www.argos.co.uk/product/4153915 ) consume around 2.4 or 2.5kW. Electricity typically costs around 12p per unit in the UK, so that means that, while operating on full power, such a tower heater costs around 30p per hour to run. However the actual cost is likely to be quite a lot less if the built-in thermostat is turning the heater off for much of the time. How often that happens would depend upon the ambient temperature and the size of the space being heated, so it's impossible to give a precise answer.