I always use the bus (3€, as I'm over 65, 6€ otherwise), unless I'm in a hurry. There's a unit price of 38€ for the cab (50€ after midnight), plus tolls, but this is still "cheap" compared to most cities in Europe and North America and thinking of the distance. Anyway, from Syntagma to Thissio it should not be more than 3€, plus a small surcharge for using it after midnight; certainly not over 5€.