Average is probably under $2. We spent $5. Remember, favors are never required. I'd rather get nothing than get something that is cheap or tacky simply because the host wanted to give something.
Food/edible favors are always well recieved and can be relatively cheap. Make sure they're professionally prepared so it's obvious to your guests that it was done in a clean, sanitary manner. No one knows the condition of Aunt Suzie's kitchen, no matter how good her home made cookies are.
Re: Money