Do you already have any code, or are you expecting the developer to write everything from scratch?
Are you looking to recruit a team of developers? It's almost certainly too much work for just one (if you want it done anytime soon), but grabbing random people off the internet usually doesn't work. Consider trying to recruit people that have worked together before, or at least know each other.
What generation do you want the graphics and battle mechanics to be like? You say "2D", so I'm guessing FrLg/RSE-like?
I think $250-$750 is way too little (assuming that's you), so hopefully that's just a random number and not what you're expecting to pay. Even entry-level web development jobs here in the UK start around £100/day, and it's obviously going to take more than a week to build a whole Pokémon engine (which is why I hope you already have code—in theory someone could adapt Essentials or another engine to have the online features you need in a reasonable amount of time).
Have you considered buying a copy of the code off the PokéMMO team, and recruiting developers to work on that? I expect that would work out cheaper, and it's a much safer idea because there are lots of developers that won't be able to write an online game (in particular, with regards to security/stability—bad network code is very hackable).