Just thought I would Add; almost nothing I have read or been told was right when I overstayed. (I left on Wednesday 1 April 2015) You cannot even check in for your flight until you have paid. You cannot pay at migraciones. And you need about an hour extra at the airport.
You need to go to migraciones,they will issue you with the papers. Then you need to go to the Banco Nacional to pay and get a receipt. hen with your receipt and papers you need to return to migraciones to have it all Validated.
Finally you can check in, then immigration will check it l again, before writing the number in your passport and finally allowing you to leave.
To read the full story click on my blog.