Electric will vary, you won't have the heater running much in summer and may not in winter either, depending on if the tank is in the house or a garage, the temp of the water 76 vs. 82, and how warm your house is. I have my 125 set up in the bedroom at 76, the bedroom is one of the warmest rooms in winter and coolest in summer and the 250w heater does fine. The heater hardly ever seems to be on, but I sometimes see it on.
For filteration costs, again it's going to depend on what you decide to use. I ran the 125 with 1 Emperor 400 and 2 Aquaeon 75 filters. It was plenty for that tank, and once a week one filter would get rinsed out so there was never a drop in bb. Figure those filters are running 24/7 at 'X' wattage. I still haven't tested them to see what they pull on watts but you get the idea here. There are lots of alternatives too. A sump, canister, or sponges. Wattage will vary with all so research it a bit. I have switched my filters around again, the Aquaeons seem to be hit and miss, I have had two get bad impeller vibrations after awhile and had to warranty them out. They were of the old 55/75 variety, the new 75 seem ok - so far.
Then comes lighting, how many watts are pulled by the bulb and how many bulbs are you using? I run a 4' strip on my 6' 125. It does stay a bit darker on each end but there also is never an algae issue either. I have a twin setup but I only run one 6500k bulb a 40 watts for 10 hours a day. It grows decent algae in the middle of the tank where I like it (my pleco likes to snack on it and the platies do too).
Finally, if you are breaking down one tank, say a 40 gallon, and replacing it with a 125 gallon - your not adding the full cost to what you are billed now, you will be subtracting the current usage too. So if you are using a 24 inch strip light which is usually either 35 or 40 watts you almost break even there. If you are running an Emperor 400 on the tank now (which is what I ran on my 40), I would move that to the 125 and add another filter source too, so you only add the additional filter costs. I would look at either a 250 or 300w heater for it, higher if it's in an unheated area.
Other costs will be the increase in water usage for water changes. You may want to check your water provider to see what (if any) options are there. We have billed tiers and stuff, it varies summer to winter etc..
If you want to make a great investment for the tank I would recommend this - buy a hydro pro 5 sponge with a powerhead and the flow attachment under $40 shipped (
www.jehmco.com/html/hydro-sponge_filters.html items HS-FP5C100 with HS-FPLN). This has two uses, first - the sponge is great for beneficial bacteria and can really help out if there is a power outage, second - you can push a hose onto the output end of the powerhead and simply pump it out of the aquarium for water changes. We run all the old fish water out to the flowerbeds and trees in the yard and they have done really well even in the 2 months of over 100 degree temps this past summer.