It depends on where you practice, what specialty you practice, how much you practice (moonlighter policies are sometimes discounted because of the relatively low number of hours worked), and whether that practice includes certain high risk activities.
When I was first out of residency I started moonlighting for an anesthesia group that was always short. Their carrier apparently wouldn't cover as a locums someone who was going to be a long term, ongoing frequent flier, so they told me I needed my own policy. They got their other locums coverage through an agency which provided coverage to visitors. I had contacted them directly looking for a job, which they loved (no cut to the agency), and it was a good deal for me too since my hourly rate and call stipend was higher than what some of the other locums were getting. Win-win except I needed to get my own liability policy.
I obtained a discounted / part-time policy through The Doctors Company (thedoctors.com). In California it was a $1M/3M claims-made policy and the premium was reduced so long as I was working less than 1/4 time at that job. My first year premium was about $1600. Annual premium gradually increased to about $3800 at maturity at 5 years. Presumably it helped that I had a clean license and malpractice history.
After I PCS'd to Virginia, for about a year I couldn't find a moonlighting gig as good as the old one ... so I actually took leave a few times to fly back to CA to work at the same place. Recently I found a moonlighting job locally that is comparable in pay and work environment to the CA job, but obviously the commute is shorter. They were able to transfer/reissue the policy to cover my work here. New policy is $2.25M/6.75M and the annual premium is about $4700. Still covers the CA location too, should I go back again.
Eventually, because it's a claims-made policy, I'll need to buy a tail (or nose). They have quoted me 240% of the final year premium for that tail cost.
They have been very good to work with as a military physician. When I was deployed, they suspended the policy premiums while I was gone and let me resume them when I returned, no gap in coverage.
I'm not sure how it'll work out when I leave for FTOS fellowship this summer. I'm clearly prohibited from any moonlighting, so I won't need a policy. But I don't want to cancel the policy and pay the tail yet, only to start it up again a year later. I'm hoping they'll treat it like a deployment ... it is after all military orders to a place where I can't work under the policy.
Worst case, I'll just pay the premium for the year I don't use it.