Will depend on several things such as location and operation area, pilot training and qualifications, aircraft type and such. Best approach to this is to contact a reputable insurance broker/agent and see what the deal is. Take care.
Three years ago I paid about ten grand a year to insure a piston twin, 7 seats, for 135 operations. Liability was $1 mil/$100K per seat. I think that's the FAA minimum. Insurance compan wanted 2000 total, 200 multi, 50 type to cover.
Same insurance for Part 91 only was about $2,500. That should put you in the ballpark.