Some sprites take hours of works, so $3-$5 is kind of like $1/$1.50 per hour, which isn't very much. Some spriters take less time, but then they are very experienced and you're paying for quality. I'd say probably anywhere between $10.00 and $15.00 is reasonable for a sprite, depending on how much work needs to be done. If it's only an edit, then maybe $3 or so.
But it depends on who's making them, really, and the level of professionalism. Think about it this way: minimum wage is like $8.00 an hour where I am from, and if a spriter is a professional I think that they have the right to charge more than that. Of course, it takes less time. If the spriter is an amateur he shouldn't charge as much per hour, but he/she will probably take longer.
It really does depend a lot on who the artist is, how much detail they put into it, etc...
I'd say probably this:
Sprite: $10 - $15
Battler: $25 - $35
Tileset: $50 - $80
Also, think about this - I think maia made a post where she had contacted a company to ask how much it would cost to get all the resources done for a game, and their response was about $5000.
If $2400 of that were devoted to tilesets, $1400 to battlers and $1200 to Sprites, then by my highest estimates you'd get:
30 tilesets
40 Battlers
80 Sprites
And that would be about enough for a full game. That's professional though.
In the end though, I really don't know ~ I'm not an artist at all, and so I can't really say how much is totally reasonable. But if I was serious enough about getting Custom Resources and I wasn't poor and though it a waste of money, that's the price range I'd think the person I get was entitled to.