Hi Jo, I don't really find Kefalonia too expensive, although it's like everywhere else in that if you get off the beaten track a little you will find prices a bit cheaper.
I was on the island in June (for my 8th visit) and paid around 25 to 35 euros for a 2 course meal with wine and a couple of other drinks. Mythos, i paid anywher between 2 to 3 euros for a large one. Spirits, expect to pay around 4 euros for a VERY large measure!
I haven't actually stayed in Svoronata but it does look like a very nice resort. As miziros says, you really must hire a car to explore this wonderful island. The scenery will blow you away. Enjoy your stay!
John