my electric bill barely went up. one filter, one 300w heater and a single/double 40w tube light (t8) does not make that much of a dent in an electric bill. once i have my 5x2x2 + sump up and running im interested to see how much thats going to raise my bill.
you only really start seeing the effects when you have multiple tanks or use pond pumps to move water around your system. some pond pumps have some crazy wattages compared to their aquarium counterpart.
you can save money, quite a bit as well, by doing little things to reduce your bill. such as switching your multi-plug extension, which has your dvd player/tv/ps3/surround system plugged into it, off at the mains when they arent in use or you go out (even for just 5 mins). Getting the multi-plug extensions where each socket on the strip has its own swicth so you can switch off what you arent using.
you would be amazed at how much electricity you burn through standby. we saved about £5 a week just by doing what ive mentioned. switch to all energy saving bulbs, there is another couple of pounds a week. it all adds up quite quickly.
i very much doubt you 75g tank has caused, at worst, anything more than a few dollars extra per month.