The Indiana Fever averaged 4,066 fans per game at an average ticket price of $60 (although that price has reportedly already increased because of Clark-mania) for 36 games.
The San Antonio Spurs average 18,248 at an average ticket price of $70 for 82 games.
Plus, the WNBA' s expiring television contract is for $60 million per year, while the NBA's expiring television deal is for $2.6 billion per year (and they're reportedly seeking around triple that for their next contract).
Simply put, although they're both playing basketball, they're not in the same economic niche. Your paycheck has precious little to do with your value as a person, but a whole lot to do with the value you provide to others, and however one feels about the fact, women's basketball does not at this time provide as much value to consumers as men's basketball.
That value is definitely growing, and may continue to grow. But that's where it's at right now.