Unfortunately, I can't give a straight answer to this, but I can say that the risk for property damage is low-moderate depending on where you operate. Assuming you use a good parachute, the chances of damaging someone else's property is pretty low. If the payload lands in a tree that has to be cut down, that may be considered a loss to the land owner.
The risk is higher for insuring the payload itself, as losing a payload happens from time to time. I'm guessing that an insurance company that would insure a balloon payload would charge a significant portion of the total cost of the payload.