It's a good idea. I would think that the plumbing changes in your house would be the biggest cost.
You can get a few tanks like this even if it just for flushing toilets, garden watering and car cleaning.
http://www.ebay.co.uk/itm/IBC-TANK-1000-LITRES-WATER-TANK-/251127963557?pt=UK_BOI_FarmingEquipment_RL&hash=item3a78649fa5The pumps don't cost the earth, but ensure you research the specs on pumps. The power varies and some are better at sucking whilst other blowing out the other side. Excuse the innuendos

Point being, is location of pump in relation to tank is important
I have family living in Spain who did what Varche wished he had done. They have a 45,000 litre tank under the garden. Reason for this is no mains water in the isolated area and they get water delivered by lorry like we have heating oil delivered. Their pump is inside the house, so obviously a good sucker - 2 pumps run the whole (big) house.
In your case, even if you were to do a simple setup of tank, pump and filtering system (very important) for the basic needs already mentioned, you'd save quite quickly.
It is what I have started to do. Have one tank and pump so far. Plumbing for me is easy as I can access most in the cellar.
I have already started using the rainwater for the garden. Collected 1000 litres in a a very short time in the very wet season so all plant watering this summer has been free.