One backend with two MIGs with different regions, asia-southeast and europe-central.
When I want to calculate Cloud Load Balancer in GCP, it asked me to specify the REGION even though, I'm using GLOBAL load balancer.
Please note that the Global Load Balancer route to the closest region to the user based on IP address and it seems the 'global' region only applies to Forwarding Rules. If we add it to the region selector, "inbound/outbound data" can't be estimated.
You can follow the GCP calculator where there is a checkbox available for “Global” which will be applied only for Forwarding Rules, to use 'global' region.
As you have two different regions in your environment, you can calculate the price as per the prescribed region accordingly as “Global” refers to Forwarding Rules only. Also, please note that outbound data needs to be defined per region.