Added total BT to max firewalls in 1 go

FIREWALL.md

@@ -41,3 +41,7 @@ It costs 150 BT to upgrade from 0 to 4
 10. 1536 BT raising regen to 10
 
 It costs 3069 BT to upgrade from 0 to 10
+
+### Max
+
+It costs 9927 BT to upgrade all 3 firewalls!