That kind of impromptu transport up there can work....or maybe it won't this time. (which is why I discounted it ever working reliably for me when you first posted) Too many uncontrollable factors to make firm plans on; need to have the flexibility of situation to be spontaneous. (And the R$500 you quote could be burdensome for a single traveler without a group on the same precise schedule to divide cost with.)
For example, the car that was supposed to take me from São Luis to Santo Amaro, one used regularly by someone in the house where I stayed, did not show up as arranged (apparently broke down). No phone call to inform. Stuck trying to make last second plans, luckily in São Luis and not in the middle of nowhere a truck ride away from the person with the connections.
To depend on everything going smoothly up there, locating the right person in a timely manner, making some connections in minimal time, particularly in order not to miss an international flight, is foolhardy.
@andrew, musta been a really huge hostel
(Note: When I started giving info on this area on all the Brazil travel forums, I was pretty much the only one posting firsthand info.)