If you overstay your Visa, the fine is 50 pesos but you must exit the country. You can also return the same day and receive a new 90 day Visa. Or you can renew your Visa for 90 days at the immigrations office in Buenos Aires for 100 pesos. The point is, you have to either pay the fine and exit the country, exit the country before your 90 day window, pay no fine, and return, or extend your stay at immigrations headquarters in Capital Federal. The folks down in Tigre told me I would have no problem overstaying my Visa, pay fine when I exit, and return same day for new Visa. In fact, that was the recommendation rather than paying for an extension. When I told them that I had "heard" it could be a problem later on, they smiled and said "no problem."That seems to be the general consensus of the people I know and have asked.