Providing a numeric analysis of the trade slider is an intricate task, since the policy features several intangibles like espionage's effectiveness, embargoes and the "trading in" modifiers, among others. What follows is an explanation of each item featured by the slider, along with practical considerations. A game about strategy is a game about choices — I am hoping your conclusions will differ from mine so that our strategies should diverge.
Full mercantilism: +5%
Full free trade: -5%
"Trade Worth" represents but the old "global trade income modifier". As the previous name was misleading it was relabeled. "Trade worth" increases or decreases the price of each
production unit. As is demonstrated this item bears two effects: one which pertains to trade income and another related to production income.
Flanders produces two units of cloth, priced at eighteen ducats: its trade value would thus amount to 36 ducats. Full mercantilism would increase that amount which would then reach 38 ducats. Since trade value is multiplied by efficiency, to derive provincial production income, "trade worth" is actually increasing production income. Yet, the trade value also dictates how many ducats one province contributes to its affiliated centre of trade — thus "trade worth" also increases trade income.
While a difference of 5% might seem inconsequential (especially in the above example) one needs to bear in mind that the same increase is featured in every
owned province. The difference between a +5 free trader and a -5 mercantilist is comparable to having an additional
marketplace in each province.
Unless a monopoly is set up a nation trading in its own centres of trade will never collect the full trade value as income. Even when one reaches a monopoly trade efficiency will usually allow one to rip a portion of the entire trade value, due to maximum trade efficiency being lower than 100%. This proves how "trade worth" affects production income more than it affects trade income — mercantilism thus benefits those who seek to maximise production income. The advantages the modifier provides are proportional to production efficiency, as increasing the latter will lead a former's increase.
A free trader will be enticed to trade in mercantilist centres of trade, due to their higher average trade value.
The most evident changes pertain to compete chances: mercantile nations no longer have a malus while trading abroad, although infamy retains its nefarious effects.
mercantilist nation, that owns every
province trading through a given COT, will count on 100% base compete chances in that COT (that's a stock 25% and 75% provided by mercantilism) — in practice that's a rather difficult scenario to achieve as centres of trade will generally include foreign provinces.
By comparison a complete free trader will have only 25% compete chances from owning every province in the COT — still an advantage.
At the beginning of the grand campaign it will be very hard to maintain nothing more than two merchants in trade league centres of trade, as they all are mercantilist (but Novgorod) — unless one is a trade league member, of course.
The old "Weaken Rival League" spy mission used to decrease diplomatic relations by 25 between members: that's largely irrelevant for most purposes. It was changed so that it decreases the compete chances bonus trade league owners derive from their owned provinces share.
There now are four sources of spies: max mercantilism (+1), max plutocracy (+1), espionage (+1) and "Enlist Privateers" (+0.3). The number of spies gained isn't abundant and that stresses mercantilism's advantage in this context.
Likewise merchants were, overall, reduced in numbers although previously unavailable ways to acquire them were introduced. The "Sund", "Bosphorus" and "India Route" modifiers were changed so that they grant +1 merchants (they previously provided "global_trade_income_modifier" which drastically increased production and
trade incomes in every province). The base cost of sending merchants has been increased (by one ducat), such a change favours free traders who are able to maintain several merchants with fewer costs. In spite of the diminutive base increase (+1 ducat), the actual effects are remarkable when distance and slider modifiers are taken into consideration.
Lastly, trade range has been overhauled. The range provided by trade advances has been decreased in unison with a revamped "Merchant Adventures". The previously overlooked national idea now grants +50% trade range, which coupled with free traders +25% range allows them to trade mostly everywhere in Europe. As a mercantilist Venice in 1400, for instance, you wouldn't be able to trade in Anterpen or Novgorod before having picked "Merchant Adventures" or having significantly invested in trade research or free trade.