Sorry to necrobump, but as someone who's halfway through med school and really doesn't feel like putting life on hold for another 5+ years, I'm wondering how feasible is it to moonlight for an extra 100k, or even 50k a year? It's always been down to gas and EM for me, even before med school, and this might just tip the odds in EM's favor. Fickle maybe, but it's the truth.
First you need to get into a program that either allows moonlighting or whose PD and multiple attendings are willing to lie about it. You'll probably need 3 letters of rec from attendings to moonlight.
(At the end of the day, this will be the biggest hurdle since for most of us mere mortals, which EM residency you match into is basically a crapshoot. This is also the most important reason why you shouldn't choose EM over gas for residency moonlighting potential, unless you only plan to apply to the EM programs that allow moonlighting, which would be a silly decision.)
Then you need a medical license in all the states in which you want to moonlight as well as DEA number(s). Pretty easy assuming you've finished intern year and went to a US medical school. If you're FMG, might be harder.
Assuming you can do that, $50--100K/y is quite feasible, especially if you are OK violating duty hours and either hiding it from your PD or your PD is complicit. (I did not do this myself and am not recommending this, mind you... I'm just saying. You will get in big trouble if your PD cares and catches you doing this, since the PD will get in big trouble if ACGME catches you doing it. I think most PDs probably care quite a lot about this.)
The second-biggest hurdle to making the big bucks is that for various silly reasons, your residency work schedule for each month will likely be published *after* the schedules for your moonlighting places have been published. So you risk being double-booked on your moonlighting shifts. Having the chops to arbitrarily trade shifts with other residents as well as being willing to violate duty hours (I am not recommending this!) goes a long way. As I mentioned in my post above, most residencies also have a reading month in the final year during which you can basically go wherever you want and moonlight a bunch.
I went to residency in the northeast and moonlighted at one local site for $130/h and another for $170/h and then at my attending job in TX for significantly more. Pulled ~80K in R3, much of it tax-free directly to my retirement accounts. Could easily have pulled six figures if I was less scrupulous about duty hours and didn't land a girlfriend halfway through R3.
BTW, if you want to FIRE and start building your nest egg a little early, this is a minor argument in favor of signing the early attending contract as an independent contractor. Then you'll have access to your very own 401(k) and cash balance plan that you can start filling up in residency, both with the sign-on bonus and with possible moonlighting at your attending job and any other IC gigs you can land.
Good luck!