This week, our water bill came in. it was for five hundred dollars. i freaked out, of course.
a bit of elimination (rimshot) (double rimshot) pointed to the ancient toilet downstairs, to which I have done various maintenance things over the years, including reseating it and replacing the flapper.
The primary culprit was the old and corroded refill valve, which appears to be original 1967 vintage. So I swapped that out for a new $10 refill assembly with a leak-sensing refill feature that would prevent the tank from refilling if there was a drainage link.
Sure enough, there was a leak in the base of the drain which slowly would add the tank water to the bowl. so i had to replace the drain valve assembly.
This involved taking the back tank off the toilet and jerry rigging a range of bolts and washers to successfully implement the tension join at the base of the tank. This took three trips to home depot and I am awaiting the verdict on the most recent install.
so the non-labor repair costs as of this moment are about $30. a whole new unit was $50. they also had a low-flow model with a city of seattle $30 rebate for $88, but it was out of stock.
i think it is likely that if this install attempt fails, I will just have to go with a new unit.