There are several companies offering day trips from Athens to Delphi, prices varying from €80 to €100. To save money, you can easily organise your own day trip utilising Athens long distance buses. The journey takes about 3 hours. One-way fare: €15,10 (return ticket also available). Seats are numbered.
Buses leave several times a day from Bus Terminal B. Finding this terminal is a bit tricky, as it is not on a main road and local buses do not stop there. As shown on this map, you can take a bus to Liosion Street and get out around number 260. Then walk to parallel street Agiou Dimitriou Oplon. Use Google Maps Public Transport to find buses to 260 Liosion St. See also Athens Guide: Bus Terminal B.
It is recommended to buy bus (return) tickets to Delphi the day in advance, so you are sure they are not sold out and that you can find the bus terminal on time.
Daily times to Delphi from Athens are: 07:30, 10:30, 13:00, 15:30, 17:30, 20:00
Return: 05:30, 09:00 (not on Sunday), 11:00, 13:30, 16:00, 18:00
Times may change