@Snarp—Rainwater is a good use for this, and some people have catchment basins on their drains for at least watering the lawns.
Getting that water pressurized to do toilets and such is difficult on a small scale, you could either use an accumulator on the line (along with a pump), or put a tank on the roof (or in the attic!), that you’d need to pump into.
Anyway, these are all under $500 options, and would easily save you that much in water bills depending on your area & amount of rain. Were I to do this, and capture 50% of my roof area into a catchment, I could theoretically capture 20k gallons of water a year. If I were able to hold all of that water, and use it as greywater in my toilets. In theory, I use around 10k gallons of water in my toilets each year, so that would be my savings (theoretically).
10k gallons is around $75 a year, so my $500 pump/tank system off my roof drains would be paid back in around 6–7 years.