I presume the EUR360 (AR$4800-5000 pesos?) was for a private service, so would certainly be more than MTL shared services.

Have you looked at simply flying there? AR flights are not very expensive (about the same as the bus fare quoted), but it looks like you'd be stuck choosing between 07:20 EZE departure (almost certainly too early for you to connect) or a 18:30 AEP departure (would need to transfer downtown with your things, probably have some time to kill, and then get to AEP for a 50 minute flight).