what price for weekly travel card for train from birmingham international to london euston?

try http://www.thetrainline.com

An adult 7-day standard class season ticket varies depending on the operator and route used.

London Midland trains only, to London Terminals (inc Euston): £120
Any service shown in the Journey Planner, to London Terminals (including London Midland and Virgin): £212.30
Any service shown in the Journey Planner, to London Zones 1-6 (including London Midland and Virgin): £225.60