Kalimera, when booking accommodation as a solo traveller, you usually pay a higher price all over the world, not just in Plakias. If you book accommodation for two, you simply split the cost (just like with a hire car ? it?s always cheaper that way).
There are a few places to stay (Stella, GioMa...) that rent out rooms quite cheaply to solo travellers, especially in the off-season.
The best thing, of course, is always to view the accommodation in person. Otherwise, I?d have a look on Booking.com.
Ta Leme, kv