The situation is more complex than just a percentage penalty. If you didn't declare anything and received money, the tax authority can charge back taxes (progressive, from 7.5% to 27.5% depending on your bracket) PLUS a penalty for failing to file. That penalty usually ranges between 75% and 150% of the tax owed, but there's a minimum penalty that's around R$ 165 or more. The problem is the longer you go without declaring and the larger the amount you omitted, the heavier it gets.
What helps in your situation is that they found out about this while you're still alive to correct it, basically. If you go to the tax authority now and file an amended return, explain that it was unintentional omission (and it really does seem to be the case considering it was informal freelance work), you have a real chance of getting a significant penalty reduction. There are people who've gotten 50% or more reductions when they regularize in good faith. If you wait to be caught, then things get ugly because it looks intentional.
My suggestion: don't panic about this. Get in touch with an accountant or even the Federal Tax Authority to clarify exactly how much you received, in what periods, and file an amended return as soon as possible. The cost of regularizing now is way less than what you'd pay if you wait and get hit with an official assessment later.