I have a two person household, a pool that I have to top off with around 500G a week in the summer time (evaporation rate is crazy during hot summer months), a 770G aquarium that I change out 200G per week on and my water bill has never been more than $50 per month and on average it's about $35 per month.
I get charged about $6 per 800G used.
To get a more precise estimate, if you find out what your municipality charges you can try to do the math.
- Shower on avg. = 2.5GPM: If you take a 15 minute shower every day = 1,125G per month for showers
- Water changes on aquariums: Doing weekly 20% water changes on 235G = 188G per month
- Laundry Machine: Can use around 40G per load- One load per week = 160G per month
- Dishwasher: ~6 gallons per load- One load per week = 24G per month
- Toilet Flushing, hand washing, teeth brushing etc, add another 300G per month
Total ~1,800 Gallons per month
The big variable is going to be your garden (if you have one). Lawns can really drink up a lot of water in the hot summer months (my irrigation is from well water, so I don't pay for that, otherwise I think my lawn would easily double my water bill in the summer time). I'm considering removing my lawn and going with native landscaping because even though I don't have to pay for the water, I still have to pay for the electricity the well pump consumes.
Congrats on your new place. I would saw go ahead and add that 125G...it's going to have a pretty miniscule impact on your water bill and if you set it up using efficient equipment, it will have a small impact on your electric bill.