According to ESPN.com, the Sonics are the 33rd-best franchise in sports. Their best ranking came in "Bang for the Buck" (i.e., wins per dollar), while the worst ranking came in affordability (i.e., how much it costs to go to a game). Among NBA teams, the Sonics finished 11th.
It's more interesting if you break it down by city. The Sonics easily outdistance the Seahawks and - surprise! - Mariners. The M's were undone by their bloated payroll and last year's craptacular season. In fact, the M's finished 90th in "Bang for the Buck", while notching a #4 spot in the stadium category. Oddly, Paul Allen's Seahawks ranked first in ownership among the 3 teams, with the Sonics 2nd and M's 3rd. I don't think Bob Whitsitt had a vote, though.