Last time I overstayed, the fee was 300 pesos, or $100 US for non merco-sur citizens (better to pay in pesos.) They've started collecting money any way they can, so they actually count the days. Last time I went to Colonia, the immigration agent actually counted the days on her fingers.
Also, Americans/Canadians/Brits need a 'visa' now to stay in Argentina, which cost me $140 US when I came in this past January. (It's basically a $140 sticker which they put in your passport.) If you missed that, you'll probably have to buy it on re-entry.
I heard a rumor that overstaying 3 times will get you 'banned' or locked out from the country - does anyone know if this is true? I really didn't want to chance it.