The brokers I've spoken to Ontario were able to generate a new "Airsoft" Policy which uses the same properties taken from the current existing Paintball policies. When setting it up they are going to ask you specific questions such as, how many individuals will be on the land at any given period for coverage, eye wear, face protection, projectiles, parking of individuals vehicles, etc.
Price depends on a LOT of variables, it can scale quite a bit. Where it's located, what it is zoned as, how much liability coverage you take out. There are no month to month rates available that I know of, so you need to purchase it as an "annual" policy (At least from my experience, this may depend on the company providing the insurance)
1 Million liability is like the bare minimum. My recommendation is to get the best recommended coverage possible to make sure you're protected.
Just start phoning up some brokers and fish around for quotes, they are competitive with each other, its definitely worth doing the research on each offer.
Hope that helps!