The value of chillies you could grow from five litres of water it would be quite small because chilli plants need considerably more water than that over a full growing season.
As a rough UK estimate, suppose a chilli plant ultimately uses around 20–40 litres of supplied water while producing its crop. If a productive plant gives 300–600 g of chillies, then 5 litres represents roughly 40–150 g of chilli production on a simple water-allocation basis.
At supermarket prices of roughly £8–£12 per kg for fresh chillies, that’s approximately £0.30–£1.80 worth of chillies.
So for your Circleism-style calculation, I’d use about £1 of chillies per 5 litres of water as a simple memorable estimate.
Rainfall, pot size, variety, temperature and recycled/drainage water can change the figure enormously.
