It's hard to give an "average". The cost will vary widely depending on the quality and detail you want and who you hire. A logo design could be anywhere from $10 to $10000
+. But, assuming you want something pretty cheap, you could figure on $100-500 per piece. The cost for the banners may also depend on complexity - 10 characters take a lot longer to paint than 1 character and some clouds. It helps if you have a clear idea what you want. If you have no idea, you'll have to go through several iterations with the artist, and you will either pay significantly more for that, or your artist will hate you.
Or, if you don't much care what you get, you just need
something (random app-store knock-off?), that makes it easier/cheaper.