The cost of using the old vs the new 1.6 gal depends upon the cost of water in your area. Using the $75 as per the previous response, find the differance in galons per flush, multiply by number of flushes per day(week) and multiply by 355 (or 52) to find galons per year. Multiply by the cost of water in your area (watch your decimal point). The resulting number is the cost of keeping you old comode. I would suspect 3 to 10 years before any payback. There is also a conservation viewpoint, save as much water as you can.
