When I checked the price of the transfer and compared to the price of the rental car, it was definitely cheaper to rent.
Even if we don't use the car when we are in Gramado, we will still save money for renting to get there.
Rental: U$ 117 (using kayak.com)
Transfer out using private car - U$98 (R$280)
Transfer in using inter-city bus U$ 13 each (R$ 35each)
However it could have been cheaper if we could get a transfer out with VMC Turismo, at R$55 each (That's U$20 each - 20+20+13+13=U$66) it could have been a better idea.