Yes, that would be quite reasonable because you'd be deducting the Airbnb fee and you'd presumably be paying cash. Does Airbnb charge the 21% VAT tax in BA? I don't know. If yes, add that into the mix. Also, paying with new $100 USD Benjamins would strengthen your bargaining position. If all of the above are true, if I were you, I would start off with a bid for a 20% discount. If the situation were reversed, the Argies would certainly ask you for it.
If you like the place, tell the owner your intention quickly so that he/she doesn't rent it out.
Another factor will be to understand if the owner is the one listing the property on Airbnb. Many owners here use a manager as an intermediary who lists the properties and deals with tenant issues like waiting for the tenant to arrive at 0300 and repairing trashed apartments. Don't pay a middleman if you don't have to. Good luck.