No player in the D-League makes $30,000 a year. The competition in general is not as good as the top European leagues, the Spanish, Italian and Russian leagues. In fact, a good case can be made that it's about the same as the Australian League. Julius Hodge's stats in Australia last year were an almost exact match to his D-League stats.
Players go to the D-League not to make money, because 1) there is none in the D-League and 2) if they were any good, they would be making a lot more in Europe or even Australia. They go there because the games are heavily scouted by the NBA and dozens of players get called up every year to the NBA where they make real money.
So if you see a veteran player like Eddie Gill--or a successful European player like former Net summer leaguer Josh Davis--join a D-League team, they want to get noticed, want to get called up. And although the Nets can only send their players down to one team, their D-League affiliate, the Colorado 14ers, they can pick up players from any team.