I don't know if it will be economical or not, but it might be worth considering if you have a water scarcity issue. If you are on a cistern, or have a well with restricted capacity, it might make sense.
I think the elements to consider would be:
1. A storage tank to capture greywater from the shower drain.
2. An overflow that diverts the greywater to your regular drain once the storage tank is full
3. Some kind of filtering to get hair, bits of soap, etc. out of the system
4. a pump connecting the storage tank to the toilet tank, triggered by the float arm. I would bet you could get a small aquarium pump that would do the job.
5. Unless you keep an eye on it, you might want to add some kind of fresh water intake to the storage tank, controlled by a float switch. This would have the storage tank topped up by fresh water if it gets too low. This would prevent a situation where you have a lot of intensive toilet use (like a party) before the shower drain tops up the tank.
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