I read at the weekend he has a 30m euros buy out clause, but that goes up to 60m euros if Athletic wanted him!!! I also read that he is from the La Rioja region so wouldn't be available for Euskal Selekzioa like Griezmann presumably!!!
In normal conditions he could be eligible for both.
It has to be said that the euskal selekzioa never calls players who aren't basque born (or basque lived since small kids as De Pedro, Valverde or Amorebieta). People as Aranzubia, Griezmann, David López or Ezquerro never played for the Euskal Selekzioa even though they lived for many years in Euskal Herria.
The reason is probably that it becomes a bit complicated to know where you should put the limit as EH is not an official country and therefore these players can't get the basque nationality. If the Basque NT happened to be official these players could logically be called in case they get the nationality and wanted to play.
Right now Valverde has no less than 37 players to choose from next season...current first team players, players returning from loan and players promoting from BA:
Óscar de Marcos - Xabi Etxeita - Aymeric Laporte - Mikel Balenziaga...