Slow cookers usually have two or three power settings. They vary between models but the following are typical examples:
A 'Low' setting might be around 180W. That's what you'd use if you were going out to work at 8am and returning at 6pm. You'd use 1.8KWh of electricity, so (with a typical price of 10p per KWh) it would cost you 18p to cook your meal
A 'High' setting might be around 360W. That's what you'd use if you were only going out for half as long but (with double the power cancelling out half the time) you'd still pay 18p.
I've never used a slow cooker but I've often left casseroles in the oven, at Gas Mark 1, while I've been out for the day. They're always vastly better than casseroles cooked for the more usual times of, say, around 45 minutes. (For example the meat just falls off from chicken breasts and is full of flavour. Even the cheapest cuts of beef come out beautifully tender. Marinaded meats absorb loads of flavour).
Asda were recently selling slow cookers for £7. They seem to have gone up since then (to £12, I think) but they could still be worth trying.
Chris