Last time we took the bus directly from Izola and it cost 10€/head, one way. As tourist season is over, we will need to get the bus into Koper (1.80€ one way per person) and then out again (passing Izola as we go) but due to the additional distance, we pay 13€ to Rovinj. That would make it 118.40€ for the four of us- return.
Wifey checked GoOpti and it comes to a straight 120€ for four, return and we get picked up on the doorstop with a direct trip. Clever wifey.