There's a lot of survey companies out there. If you sign up for multiple sites, the volume can end up being enough for steady part-time income on your time off (though volume probably still not enough for replace a full time physician job).
Availability is largely specialty dependent though. Sermo seems to largely target an international physician audience so their pay may be more on par with physicians in other countrifies, but most of their surveys pay low for the amount of time spent by U.S. physician standards.
Pay can range anywhere from $50 to $300 per hour from what I've seen. If you can get more surveys paying on the higher end of that range, it may be comparable to pay for clinical practice in your specialty, but much more flexibility to do from home and comes with none of the liability that comes with clinical practice.