As Steve pointed out, its more likely your"e also clearing a deposit bonus on top of the 30% rakeback.
In terms of living off RB, some mid to high stakes multi tablers strike a balance of play whereby, they play x amount of tables at x stakes and though their winrate is zero or slightly minus, their hourly rate is optimum and some of the top guys are making as much as $30k pm. But you do need to be really good to achieve this.
As far as rakeback goes, your 30% deal is towards the small end of whats on offer. The big two Stars and Fulltilt won"t go beyond that afaik but lots of the lesser sites are giving 55% RB if you rake as little as $1k pm.
As an example 50 to 60 hrs per mth 6 tabling 50nl at 55% RB would net you about $550 to $650 in rakeback.