The rakeback is exactly the same % on sngs as it is on cash games. The only difference is that the rake on an sng is the tournament fee and applied just once.
At micro levels, you would tend to earn more rakeback per hour on cash games because you tend to buyin to cash games with a higher amount of money & sngs take a while to finish even when multitabling.
e.g.
> multitabling 4x$1+0.20 sngs together takes approx 40-60 minutes and you have $4.80 staked at any point in time.
> 4x tabling BSS NL5 for an hour, you have $20 at stake
As you move up the sng levels however this soon evens out.
Obviously rakeback isn't everything, concentrate on beating the levels in their own right and rakeback will be literally be a bonus on top.