How should discounts that change over time be handled?

[maxinfet]

If a discount changes over time and you want to add a permanent unit to build how should it be handled? I think the best example of this is Indians which get a discount on villagers starting in the dark age and it gets progressively better as the game goes on.

[posixpascal]

This feature is hard to visualize. I could let the user check each age-upgrade he is interested in and show the amount of villagers needed per selected age. Do you have a design solution in mind?