You can extend your visa at the Federal Police. It's around 60-70 reais. Let me know if you're interested, and I can explain it more in-depth. I've done it before, and it was quite easy. If you take this option, I'd suggest purchasing a flexible ticket through GOL that will allow you to change your date of travel without a fine. You will want to purchase a ticket within the validity of your visa when you travel to Brazil, and subsequently change the return of your ticket when you go to extend it.
The visa can only be extended for a max. of 6 months.
If you overstay your visa, you are charged $7something reais everyday for 100 days. The maximum is $800something reais, which I believe you'd pay when you come back. In my opinion, it's never a good idea to overstay any visas.
Brazil has serious issues with their stamps. I've been given exit stamps instead of entry stamps without even knowing it... I've met several people who've had the same problem.