Probable not that much
I don't think it drains that much, I have always left mine on since the first iPhone. The few times I didn't use my phone much the charge lasted for days and wifi was on. I think you should worry more about location services, bluetooth, notifications, weather widget and push email.
Now if you feel like cutting it off all the time sure it will probable save a little. But if I did that I would most likely forget to turn it on when i get home and run my megabytes up on my cell plan, AT&T is pretty fast at my house so it would be hard to tell if was surfing on wifi or 3G on my phone.