The "cost" of the annulment is for the investigation. The tribunal does a through investigation to determine if the sacrament has been confected (performed) correctly. This is a lengthy process that takes a lot of time and paperwork. Not everyone that applies for one gets one and depending on financial circumstances the fee can be waved.
That being said: Yes there have been abuses to the system, but you don't blame the system you blame the individuals.
I got an annulment several years ago. I was told about the fee for the processing of the paperwork, but that if I could not afford it, it didn’t matter. They would proceed anyway. They ask those who can afford it to help offset the costs. Those who can’t afford it, simply don’t have to pay anything. At least that’s the way it is in my diocese.