We stayed in Lassi and couldn't find this trip through any of the agents there, so we arranged it ourselves. The total cost was € 250 return for a family of two adults and three children. The adult fare is more expensive than the children's fare. It takes about 7-8 hours in total using both coach and ferry. There are a few different routes that can be followed. Our outbound trip was taking the coach from Argostoli, coach boards the ferry in Argostoli and sails to Killini (Greece), then coach to Athens. The return trip was Athens to Patros (Greece) by coach, Patros to Sami (Kefalonia) by ferry, Sami to Argostoli by coach. We left at 7.15am on a Sunday, arriving in Athens at 2.30pm. The return trip left Athens at 5pm and got back to Argostoli at just after midnight. We stayed ine night in Athens, which we pre-arranged. Our operator gave us names of hotels to contact.
We were advised in Argostoli to book the return trip in Athens, which turned out to our advantage. One word of caution, if you haven't experienced the Athens bus terminal, be prepared for a nosiy, dangerous and very hot terminal. Hope they sort it out before next year's games.