That sounds really interesting! The skill tree could also be used for such a thing! (Queue my shameless promotion of my own idea! 
) Skill-Tree suggestion for 9.0 [I may need to modify the title [9.0 to 10.0] hehehe]… But I like the sound of getting one upgrade reducing the cost of another, like a bank respecting a big investor more than a little one!
Best regards,
Wise.