I'd say an hourly rate of 15-25 would be fair, but not 15-25 for the whole tileset. If you're not THAT rich, which is understandable, Maybe 7-10 per hour. You'd just have to have the tileset artist keep track of the time they work on the tileset. Giving a tileset artist something like 25 dollars for a whole sheet is really like paying them a dollar per hour, or sometimes even less depending on how long they take. Of course, make sure you get what you pay for.
For very good pixel artists, it could be $50+ per hour.