considering its more expensive to take a shower (every time you mess) than it is to flush a toilet the costs would be increased there add another shower to each day then you get the picture.
Shower uses between 8 and 20 gallons
toilet flush - 4-7 depending on toilet. (rough conversion from litres)
http://wiki.answers.com/Q/How_many_g...age_shower_use
http://wiki.answers.com/Q/How_much_w...when_you_flush
Combined with the costs of specialized expensive toiletries (though baby wipes are pretty useful) and the diapers themselves it indeed costs more money than it saves.