Unless you're really working on "no budget" you should get insurance. I have always used the good folks at ProductionInsurance.com. Since 2005 I've probably used them around a dozen times. I have never had anything but frustration when I've been forced to use someone else (usually when someone says they have a friend who can hook them up with good insurance). The reason why Production Insurance works so great is because they're completely online. If you are grabbing a last minute location, you can tell a homeowner they're insured, get online, put them on your list and print up an insurance form listing them as an additional insured in a matter of minutes… twenty-four hours a day. With other companies you can only call when they're open and then (I have gotten this before) they tell you that it will take about five business days to process. Or even better, when my co-producer got insurance through his friend, we could only get proofs of insurance through said friend, who might be at lunch or golfing, or who knows where when we needed him. Dave Morrill who runs Production Insurance has also been very helpful when I've had last minute needs with the permit office or what not. I think they're services are for films under $1,000,000 budget and that shoot less than thirty days.
The costs vary, and one of the ways to tell if a script will be low budget or not is to look for things that require higher insurance. Got people driving in cars? That's gonna cost you. Got people jumping over fences or off ledges? That will cost you too. Got any scenes on, in or near water. That will cost you as well. Obviously any stunts or fights will also create insurance issues.
You can cover most of your equipment rental with a $100,000 policy. Almost all permit offices will require a liability of $1,000,000. I also get third party damage coverage, which covers our crew breaking stuff. You will often also have to get auto coverage, in case a car does any damage. Getting auto coverage here is cheaper than getting it through a car rental place, if you're renting cars. All this should cost around 5k-7k depending on what your needs are.
I'm jinxing myself here, but up to now I've never had to file a claim. So I don't know how productioninsurance.com is when it comes to claims.
If you're super low you can fly without insurance. You won't be able to get a permit, but I assume you're off permit anyway. Most rental houses will sell you insurance for your rental at just an additional cost. Some savvy locations will not let you in without some liability. We are generally able to convince most to let us in by putting something about "Indemnifying the owner against any liability claim" in our location contract -- I'm not a lawyer so that piece of information should not be pursued without the obvious care and caution (blah, blah, blah).
One of the best ways to ensure that you don't need insurance is to hire smart people who aren't going to do stupid and careless things. There's a skill set that is vital in the film business and that is working quickly and carefully. Too often I see people who become careless as they try to move faster. These are people to avoid. Real professionals can move quickly, be careful, and still give you the highest quality work.