yes you are right !
In the end the system is that Summoner cost 2 Mana and every buff 1 Mana, so summoner with one buff cost 3 Mana, with three buffs 5 Mana (in the end The Peakrider has also three buff as the normal buff is +1 shield and he get to this +2 shield more and -1 opponent range attack).

From this Daria and Delwyn go away and one buff cost not one Mana but two (Daria) or three (Delwyn) - for me it's not logical ... ok you could say one Mana for the ability to use Dragons, but than is the question why Selenia Sky cost only 3 Mana.