I haven't run a MOOC, so I cannot comment on how much effort that is. However,having recorded many lectures at the Texas A&M television studio that are available on youtube (see http://www.math.colostate.edu/~bangerth/videos.html) I can comment on the effort related to recording:
First, it's an enormous effort if you intend to record and edit videos. Given that you're the expert on the subject, it's likely that editing would fall onto you, and you will have to estimate 2-3 times the length of the videos just for editing -- in your case, that would be 2-3 weeks.
The alternative is to just have a single cut for each video. That's the way I went. Most of the time it goes well, and occasionally you just have to start again from the top. Not all that big a deal -- if you stumble or something goes wrong, you just have to make it into a joke and move on, all while on camera.
Second, if you can't edit, you have to get it right. That means that by and large your slides have to be meticulous, and you need to know exactly what you want to say. My estimate is that it takes you 4-6 times the length of the lecture to get to the point where everything is in place, both on the slides and in your head. So, preparing 40 hours of lectures will take you 4-6 weeks of preparation. This is much more than a regular course because for a regular course you can fill in gaps from your background knowledge as you stand on the whiteboard, and you can read off the faces of your students whether you need to go into more detail or not. No such luxury when in front of a camera -- it has to be complete, and at the right level, on first take.
Third, there is of course also the time you actually stand in the studio. I've found that I record for maybe 3 hours a day, but certainly no more -- it's tiring to talk to yourself for longer. So I got 3, sometimes 4 lectures (at 30-45 minutes each) in per day, at most. But there's breaks between recording, time to set up, time to break everything down again, so you may spend 50% more time in the studio than just the time to record. A total of 1.5 weeks in your case.
All in all, it's a lot of work to record a semester's worth of material in the studio, even if (as in my case) you have fantastic studio staff doing all of the recording and production work.