How to change unit cost in-game?

Is this even possible? I want to make it so that as the player buys heros, each new hero will cost more than the previous one. Like 425 base cost, plus an additionaly 100 gold for every hero you already own.

I was thinking I would have to resort to a dialog message saying you have to pay tax for new heros. That is an OK work around, but I am not even sure how to work the triggers for that.

I guess you could do it using upgrades... upgrade costs increase with every level. just try to make the research upgrade icon look like a train hero; give the upgrade 99 levels (do you think that's enough?) and use triggers to let a hero spawn after you finish the upgrade.

Ugh. That's a smart idea but I don't want to do all that on my map. I am using every single Hero in the game, and eventually some custom ones. I just want the players to be able to buy the heros from their usual altars and the tavern.

I wonder if I could use the Charge Gold and Lumber ability to my advantage. I don't know.