but still
if you really put a pencil to it, at $500 pesos per taxi, and then another 900 pesos for the van back, that 1900 pesos, or approx $200 USD, even at $25/pp, that is quite a bit of cash for an expedition to Sayulita, plus you still have food, drinks, etc on top of that... personally i still would take the bus for $4 USD and use my $20 USD for cervesa from the local tiendas, or pastries from the beach vendors, or tamales from the beach vendors... but each to their own....
lots of suggestions