I did it once but it was based on a Brinsea Octagon 20. If you take as an example an Octagon 40 which will take 48 hen eggs which on a good hatch might give 40 chicks, they quote typical 44w for incubator and cradle. That comes to about 1Kwh per day so 21 Kwh to hatch.
I use those electric hens which use far lees electricity than a lamp I think. The Brinsea is rated at 50w so that would be 1.2Kwh per day for say 42 days although in good weather shorter than that. Also not sure you can get 40 x 6 week old birds under one. Anyway, that's 50Kwh approx.
So, approx 70Kwh to 6 weeks at approx 10p/Kwh is £7. With a 150w heat lamp, that would rise to something like £17 I think.