In the past (at least as far as I know/remember) one overstay hasn't resulted in any problem upon returning, even in less than three months. However, If you go to migraciones and get a prorroga withing the next 28 days, then leave the country and return while the prorroga is still valid, you will not get a new 90 day visa when you return to Argentina. Border agents cannot override a prorroga. On top of that you will not be able to go to migraciones and get a second prorroga.
Recently one member of this forum posted that he went to migraciones to get a prorroga (before his first 90 day visa expired) and asked about leaving and returning at some point in the near future. He indicated that the agent at migraciones told him to go to Uruguay to get a new 90 day visa and just before that visa expired he should/could return to migraciones and ask for a prorroga.
In response to this I asked a lawyer who specializes in citizenship if this was "official" policy now, but I didn't see a direct answer to this question. I do remember that he wrote :there are no rules" (and I believe it was in the same thread).
Regarding the "$7000" fee:
While I appreciate the cynicism, I wonder how it would be possible for a customs officer to pocket $2500 pesos if someone paid $7500 instead of the amount of $4500 which is still published on the migraciones website (see Ben's post #2)...unless the payment at the port of entry is made directly to the officer in cash and there is no receipt which shows the amount paid...something that should not be possible.
It is possible however, that the fee was (very) recently increased and the new amount has not yet been posted on the migraciones website. The last time the monthly income requirement for the visa rentista was increase from $8.000 peso to $30.000 pesos the change did not appear immediately on the migraciones website. One expat discovered the change only after going to renew his/her visa rentista at the then still published amount of $8.000 pesos.