DON'T invest another 50$ with the aim of clearing the bonus. It sounds paranoid, but that is exactly what they're hoping you'll do.
You need to think of the bonus as free money which it would be great if you could clear but not the actual purpose of playing. If you risk your own money to clear it, you're falling into a bit of a trap. Ask yourself what you'll do if you lose that money (which you realistically could if you have lost money with SSS in the past)? Will you deposit another 50$? I would counsel you not to risk your own real money for the chance to win 'notional' money.
If you play SSS strictly according to the charts, imho, you cannot fail to make money, long term, on the lower limits; but you do have to be very patient and very disciplined. However, I really think it's a prerequisite to be able to play SSS profitably before you consider moving on to anything else. If you can't do so profitably, there is a good chance you won't be able to make money at any other Holdem variety either - SSS is basically the start point of NL Holdem. Everything else requires more skill/decision making and is therefore less suitable for the kind of multitable grinding that bonus-clearing demands.
best of luck,
Tim