My testing does not yield these results.% modifiers are all added together before multiplying by the base cost so Iwanow is correct there.
However, regardless of modifiers unit recruitment costs never drop below 1 ducat.
Case 1: Venice with no galleys, Serf-FS 0 (0% galley cost modifier), Land-Naval 4 (-10% galley cost modifier), Expanded Venetian Arsenal (-40% galley cost modifier), Press Gangs (-50% ship cost modifier), Shipyard (-10% ship cost modifier), for a total of -110% cost modifier, builds galleys at 2.2 ducats, down from a base price of 10.0. If this was truly reduced below 0, they should cost 1 ducat.
Case 2: Same as Case 1 but without Press gangs, for a total of -60% cost modifier, builds galleys at 4.5 ducats, down from a base price of 10.0. If this was truly reduced by 60%, they would cost 4 ducats.
Case 3: France with a Barracks (-20% infantry costs), -5 Aristocracy (-5% infantry costs), -5 Serfdom (-30% infantry costs), Land (-25% infantry costs), Quantity (-12.5% infantry costs), for a total of -87.5% infantry costs, builds infantry at 2.2 ducats, down from a base price of 9.0. If this was truly reduced by 87.5%, they would cost 1.125 ducats.
Case 4: Poland with a Barracks (-20% costs), -5 Aristocracy (-30% costs), -5 Land (-25% costs), Use Polish Hussars (-25% costs), for a total of -100% costs, buys Cavalry at 4.0 ducats apiece, down from a base price of 21.0. If this was truly reduced by 100%, they would cost 1 ducat.
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All testing done in Divine Wind 5.1 (BJLT).