Here are some calculations for you.
Running a slowcooker for 4 hours - 11p (according to npower)
Buying organic veg so you can actually use the peelings to keep the stock 'chemical -free' - 89% more expensive than non-organic veg
Large silicone bag - £10 (amazon)
Slow cooker - £15 (argos)
Ice cube tray - 50p (wilko)