I tried to book Delhi-Leh flight tickets in January 2014. But the tickets were not only expensive (more than INR 14,000 per person) but also not available for dates of my choice. At that time, Himachal Pradesh Tourism Development Corporation (H.P.T.D.C.) did not open its online bus booking for the upcoming season.
However, H.P.T.D.C. is now allowing online booking of Manali-Leh-Manali bus tickets. The corporation would run 2*2 non-AC deluxe buses on this route from July 1- September 10, 2014. However, this is not a daily bus service. You can check the complete bus schedule at http://www.hptdc.nic.in/leh.htm.
From Manali, the bus starts at 09:00 am, whereas the ride from Leh starts at 05:00 am.
The cost of ticket inclusive of one night stay in Keylong on MAP basis (one dinner and one breakfast) has gone up from INR 2,200 per person in 2013 to INR 2,500 in 2014, an increase of more than 13%.
The online booking requires free registration at http://hp.gov.in/hptdc/Availability.aspx. The online bus booking software allows you to choose the seat number.