If you have a legitimate reason to sue they should be then they should be the ones paying for your legal fees.
There are so many corporations that can get away with being evil and breaking the law just because you would waste more money trying to fight them, than you would likely win.
I think that if you have what seems to at first be a credible story of wrongdoing and at very least some evidence to back it up that you shouldn't have to pay anything up front, then even if you lose maybe you pay a reduced amount of legal and court fees or maybe you still pay full fees but over time. If you instead have zero evidence and are clearly just wasting everyones time then yeah you can pay the full fees first if your still wanting to be seen in court.
But if you win? Then not only should you not have to pay up front, but you shouldn't pay at all the cost of going to courts, the legal fees or any other cost that occurred in order to bring about justice. Instead it should be added to their bill. The person being sued should have to pay for making it a legal battle in the first place.
Don't want to pay for someone else's legal fees? Don't make them have to bring you to court!