You're obviously not as old as me ;-)
25 years ago, any scheduled flight to Greece and back would have probably cost at least a month's take home pay. The price you quote today would leave most people change out of a week's take home pay, so (using the comparisons we oldies like to refer to), that's definitely cheap.
Looking at it another way, a return bus trip to my nearest large town costs about 16p per mile. If I take my car, the fuel costs are about 12p per mile. The train works out at 18p per mile. Easyjet are offering to get you to Athens and back for
under 6p per mile. So I still think it's cheap!
Budget airlines use their aircraft, on the shorter European routes, for 3 return flights per day. Greece might not seem that much further than, say, Spain, but the extra distance means that they can only fit in 2 return flights per day. That makes a big difference to their budgets, so prices are bound to be higher.
Additionally, there's not much competition on routes to Athens. (Most holidaymakers head for the Greek islands, rather than the mainland). So there's no great incentive for airlines to keep prices down. The only airlines which serve Athens, from the UK, are BA (cheapest fare �202), Easyjet (cheapest fare �178) and Olympic. (I've not checked every possible fare but most are around �240 - �250. Some are over �1000):
http://www.aia.gr/flight_schedule/european_en. htm
Sorry!
Chris