Hi saraE: welcome to the forum. You may get lucky and find someone to share that taxi, perhaps someone on your flight. There are, less expensive, ways to get to Sarria or at least Lugo but, meanwhile a couple of questions. Is 16:30 your flight landing time or your cleared Border Control, reclaimed luggage and out of the arrivals gate time? The two can be significantly different. Have you already booked a taxi? Some companies will try and fill their taxis some don’t bother. Otherwise the €150-200 fare will fall on you.
Monbus has a daily bus direct from the airport but not to meet your timings. They also run buses to Lugo with connections to Sarria