We have a caculator on this site that can help you calculate how much your electric bill will go up per month. Unless you have high power lights its doubtful you will notice anything more than maybe a $5 increase.
Mine went up $30 or so when I put my 80 gal
tank online but I also have close to 400W of light and two 250W heaters that cycle on/off aswell as a pump that pulls over 100W easy.
Other monthly costs would include salt. Depending on how much you do water changes you might not go thru salt that much. Other consumables would be bulbs every 9-12 months. Test kits, chemicals (if you dose), food.