Taken from Wikipedia, the average attendances for the five highest divisions in the English soccer pyramid (two years ago except for the EPL):
Premier League: 35,931
Championship: 17,738
League One: 7,358
League Two: 4,408
Conference National: 2,020
And it only goes down from there. In the earliest rounds of the FA Cup, where they have teams from divisions 9 and 10, attendances are usually lower than 100. In other major countries (France, Germany, Italy, Spain), the drop-offs from tier to tier are even more pronounced.
If you wanted to scale this to BB, you'd have to reduce everything by at least half, since soccer stadiums commonly have a much higher capacity than basketball stadiums.
It makes me wonder what if new BB teams started with an arena capacity of 1,000 or 2,000.