I've been researching it to try and get a better idea, and it is quite funny some of the information you find! One page just had an 'average use per month' figure (what is average? daily? twice a week?) while another lists the hours of use as TWO. Like, what crockpot cooks for TWO hours?? Argh.
I did come across this short article which expresses the issue well:
Basically, because its used for a short time, the oven can be more efficient. I think this will depend on the quality of the insulation on your oven, too.
However the crock pot has other benefits - slow cooking can preserve some nutrients better than high-temperature cooking, and it can be useful if you need to get dinner prepared ahead of time.
My crock pot is too big for my current needs - though it will be useful when the children are bigger and have growing teenager appetites - so I might think twice about using it unless I want to double and freeze.