From Bayahibe to Bavaro by bus: any advice?

You can catch a regular gua gua to Bavaro from the main road before turning towards Bayahibe. I used to catch a motoconcho to take me there if i needed to take a gua gua to La Romana....but I am assuming you have luggage?
if that is the case then i dont know...have the resort set you up with a car to the main road and then maybe a guagua to Bavaro? im not sure....

You can catch a voladora at an intersection (cruce de Bayahibe) at ISLA gas station. Express busses will not stop. You can hop on a Sichoprola or Sichoem bus to Higuey.

100 dollars for a taxi is not that much, but you should probably be able to work out the rate to 2500 pesos, which is like 75 dollars. It's about 1200 pesos from Higuey to Bayahibe or Romana (same distance) and about 1300 pesos to Bavaro.

If you have a (heavy) luggage you are better off taking the guagua from Bayahibe to La Romana, they drop you off at Sichoem terminal (if not, take a taxi - 100 pesos) and take the bus to Higuey.

In Higuey, both Sichoprola and Sichoem buses will drop you off at thye Bavaro bus terminal,m where you can take a regular or express bus to bavaro. Just ask for "parada de Bavaro".

First of all, small correction - Sichoem is actually SITRAIHR.

So, it ISLA gas station you would be looking for either SICHOPROLA or SITRAIHR buses.

The large buses have lots of space for luggage. The small guaguas (Sitraihr) might have some space...
Anyhow, you should not wait more than 15 minutes during a peak hour for bus to pass by. Just get yourself by the road and make the sign like when you hitchhike.

The price should be around 100 to 120 pesos, no more, to Higuey.

From Higuey to Bavaro around 100 to 120 pesos as well.

For the taxi from hotel... you should not pay more than 15 dollars/ 500 pesos to Cruce.

BUT beware you will need to take a taxi from Bavaro bus terminal to teh hotel (15 to 30 dollars depending on where the hotel is located) unless you will stay in one of the following hotels: Barcelo Bavaro, IFA, Melia, Paradissus Palma Real, Carabella, Barcelo Dominican Beach, Palladium, - In that case the bus can drop you off in front of the hotel (on their route).