Regardless of the time of day you decide to fly, the ticket price will remain around the same. Book flights from Londonderry to Belfast as you normally would.
Londonderry Eglinton (LDY) is the airport you will fly out of if you book a flight from Londonderry to Belfast. You should not have trouble finding flights to Belfast from Eglinton; the airport handles an average of 0 outbound flights to Belfast per day. You should also have plenty of airline options to choose from since 0 airlines have flights from the Londonderry airport to Belfast. It is only a 6.7 mi journey to Eglinton (LDY) from Londonderry’s city center.