When I purchase website I usually pay between 6 and 12 month max. This is because nobody knows where the "commercial" Internet will be in 1 year. The product cycle is so fast that what seems to be a profitable business today can be completely outdated tomorrow. The more people use the Web the shorter is the product cycle for any Web-based business. It is a phenomenon we must get used to live with when we try to invest in an online venture!
It depends on a lot of factors , the growth potential , the domains , the content , how much upkeep to the sites etc...etc... , but probably 12 months revenue is to be expected.