I think the whole issue is a giant plot hole. With that said:
With respect to how much in credits a person cost, I'm thinking along the lines that Republic credits were worthless on Tatooine, so it would first be necessary to convert credits. First of all, buying into a presumably technically illegal currency should be costly. Secondly, there are no Republic services out there what services there are are provided by the Hutts, and that has to be paid for somehow. Thirdly, people on the Outer Rim wouldn't enjoy the economy of scale that covers the Republic proper. For at least these reasons, I'd imagine that prices for everything across the board would look a lot higher to someone coming out from the inner part of the galaxy.
As for the J-type 327 starship, it was property of the government of Naboo and not privately owned, so it could easily have been extremely expensive if not unaffordable for Padmé's family. Also, Qui-Gon bartered the starship at a bargain basement price, one reason being that it didn't have a hyperdrive.
Again, I re-emphasize that this post represents an attempt to explain something that is a plot hole!