According to my electricity provider, Ergon Energy, the online Appliance Calculator states that it costs 3 cents per hour to run a slow cooker compared to 46 cents per hour for a conventional oven/hot plates. This is a handy calculator if you want to check out what some of your appliances are costing you to run. Your own energy provider might have something similar on their web site.
This note on the web site states:
I think the slow cooker will be the way for me for some time yet although on a really cold day, I'll use the oven to cook dinner (and to warm the house) but get the most out of it with getting on a batch of muffins, cookies and a cake or dessert.