Some of recent small sales:

US$ 8,888

US$ 4,000

US$ 4,650


US$ 20,000

US$ 7,500


US$ 10,200

US$ 15,000

US$ 90,000

DNJournal is a great place to learn about the value of domain names. They publish weekly and yearly sales chart. You can also look through archives of previous years sales. This site is useful to understand the domain market as a whole.