I would imagine earning potential to be vastly different from one server to the next. Even if you have a virtual monopoly on something that's in high demand, the number of people with the desire to buy it and ability to do so at any particular price point is limited. I think with larger servers you have more earning potential but would also have to put in more effort for each gil you make due to there being more competition and undercutting. On a smaller server there isn't as much gil available to be spent, but with less people competing in the marketplace it's easier to get your hands on it.