As a side note, in a bit older version of the game (when ridiculous cost of a new unit wasn't fixed) summoning menu button would just disappear after trying to summon either one or multiple characters. I believe this bug vanished after fixing the cost of new units, but I just felt like I needed to address this. 
Btw, did the cost get actually fixed? Because even though new allies's level gets increased by around +50 extra, the sum of money to summon a new character is practically unachievable. 