Only the insurer can tell you what the increased premium would be. Depends on the machine, the insurer, you, current sunspot activity, etc...
As has been pointed out already, if you're logging PIC time, you should actually be PIC. If you're going flying with the owner, with the owner sitting in the right seat (assuming a Robbo here), you're not PIC (unless you're an instructor giving instruction). If you're flying PIC, make damn sure that you've got insurance, either in your own right or under the owners policy.
The owner would have had the option to take, say, a 100-hour minimum on insured pilots, which would have boosted his premiums. More risk (less experienced pilots) = higher insurance costs. The cost difference probably ranges from a 'fair old whack' to eye watering; welcome to rotary aviation.