Ah so it works as minimum cost for upgrade.All this setting does is raise the upgrade cost by +1 Gold for each integer step taken. I took the slinger to 0 for this setting and the upgrade cost was reduced by 5 gold
It doesn't need to be here at all then, just be zeroed,, as most likely any unit upgraded to anything else even on fastest pace of Settler/Duel/Normal will have cost difference between two units, even if it would end up as 1 gold.