From the article it says parents can pay from 400-4000 dollars for a summer program. How do the kids pick which team their on? Do they compete for spots? Can someone or organization pay for a kid whose parents can't afford to pay? Are there any restriction by the NCAA or AAU as to who can pay?
Can't speak for basketball, but have a lot of experience in club volleyball. That (very broad) price range -- and even higher in some cases -- is consistent with club volleyball. To play on an elite club volleyball team (i.e., one that competes on the "open" level in major tournaments) is going to probably run
at least $2000 per season in any major city, and easily double to triple that in for some clubs. It varies quite a bit, and it also varies as to what is included for that price (e.g., travel expenses, uniforms, etc.).
Kids pick teams for a variety of reasons: the best team; best coaches; best schedule; college exposure; geographic proximity; etc. Most major cities have at least two prominent clubs (and often dozens of lower-level clubs that don't compete on a national scale). It's not uncommon to see the best players in an age group come together at one of the local clubs to create a power-house team at that age (and often a different local club has the dominant team in another age group). There are a handful of clubs across the country that are pretty dominant year after year on all age groups. For most elite clubs, getting a spot on the top team is very competitive. Frequently, every player on an elite club's top team will play in college. Most clubs have multiple teams at each age level, and it's not unheard of to have 10-12 teams at each age group. There are a lot of girls playing volleyball, and clubs are generally willing to form as many teams as they possibly can to bring in the money. Lower level teams usually don't cost as much and usually compete pretty close to home.
Clubs can offer financial aid. There are rules, and it's supposed to be equally available based on financial need. Not surprisingly, most of the kids that the clubs consider the neediest are top-level players. Funny how that works, isn't it? Technically, a kid can lose NCAA eligibility if they receive financial support that doesn't meet certain standards, but I've never heard of it happening.