I have an upstairs toilet. My main bathroom is downstairs. Upstairs is just something closet sized, with just a toilet. No sink. A light, and a very old toilet that needs replacing. It is so old it is Harvest Gold and Avacado Green. Really.
I saw this, the sink on top of the tank, and realized it would make the upstairs emergency bathroom much more functional! It is a sink on top of the tank, uses the water straight from the source, and then drains into the tank, filling it.
So not only do I now have ROOM for a sink, it is a money saver, and it’s green, too! I’m pretty happy, it seems to be just in the $100 range, which is more than reasonable to add a sink AND save water.