I read a very interesting article on Forbes listing the NBA's most valuable franchises. The article in itself is an interesting read, not just because it lists the rankings from highest (LA Lakers at $2.6 billion) to lowest (Milwaukee Bucks at $600 million) but also because it gives casual fans an insight into how these valuations are actual computed.
What I've learned in my accounting and finances classes at IE Business School is that valuations are always partly subjective. Merely taking the value of the assets for each team will barely even give you a fraction of what these teams are being valuated at, even for teams that own their own arenas (most of them don't).