Hands-down, our #1 iPad accessory shouldn't cost you more than $10. It's a cleaning cloth. I doesn't matter if it's Chamois leather or synthetic microfiber, just get something that works slightly better than a t-shirt.
Don't bother spending money on sprays or other cleaning liquids. The iPad's oleophobic-coated glass screen makes wiping off smudges easy, so long as you have a cloth handy. Besides, as anyone with a glossy laptop screen will tell you, you never seem to have those expensive sprays around when you need really them.