I'm just a hobbyist so I can't give you good price information, but I have a few comments that might (hopefully) help.

For the characters, finding a pixel artist shouldn't be too hard since that style is very common. However, you might want to get someone else for the battle backgrounds since that's significantly harder to do in my experience, and wouldn't be efficiently done in pixel art form anyway (and might be beyond the experience of a lot of pixel artists).

The overhead map might also be better as a large custom image rather than a tileset unless it needs to be modified a lot in the course of the game.

Well, I can tell you how much I would charge for such a project, but my prices may be higher or lower than others. It really depends on who you find.

For the sprite work, I normally would charge $60-$80 an hour and this would probably take 10-15 hours a character. (1hr for a character, 2hrs for animations, 1hr each arm per weapon set, all followed up by a 5hr window for unforeseen issues. Usually deducted come bill time, but I find it's good to give a buffer in the quote)

For tile sets and battle backgrounds, it's really dependent on how detailed you want your environment to be. This one I would probably quote between $80 and $160 an hour for probably another 5-10 hours, but that's depending on how detailed and difficult the client request. (I don't like doing environments that much, so I tend to charge higher there).

Your best bet would be to find an artist you like by looking around the classified section here or browsing around deviantart. Then just send them a message for a quick quote and see what they have to say. Never hurts to ask.

Good luck!

There are plenty of artists who can do all of that. Fee is negotiable in some cases. A hobbyist probably would do it for free, the experience, and resume point. A semi-pro artist would ask 30 to 60 USA dollars per hour approximately and with some exceptions lower or higher. Professionals would ask for about 50 to 100 USA dollars per hour. Keep in mind that the higher the quality of the artist, then the more fee asked by them and less opportunity for negotiating fee, though you won't need them for this game designed for a contest.