I guess most people's question on Rice Exchange is on whether we should upgrade Markets in all provinces. But obviously we at most can only have 5 Metsuke supervising towns. So most provinces would be without a Metsuke. Let's look at the no metsuke situation.
Base Tax Rate (BT%) = 30%
Number of Provinces (P) = 22, 16, 10
Number of Turns (T) = 1 to 100
Effective Tax Rate (ET%) = 20% (22 provinces), 22% (16 provinces), 24% (10 provinces)
Average Effective Tax Rate (ET-AVE%) = 20% (22 provinces), 22% (16 provinces), 24% (10 provinces)
Chart 2 - No Metsuke in the Province
Without Metsuke, the extra commerce offered by Rice Exchange is subject to the regular 20% effective tax rate in the province. And we have a very surprising result: Rice Exchange does not turn ANY extra profit after we grow reasonably large.
Extra profit we make at the 10 province case is also very small. It is a very questionable decision to lose 1500 koku now only to gain that same extra koku back 80+ seasons later. Think of what you can do with those 1500 koku per province!
I think the verdict is quite clear now so we can reach a conclusion:
CONCLUSION
(1) Upgrade a Market to Rice Exchange if there is a good Metsuke stationed in the province. Since at most we have 5 Metsuke, we will at most upgrade 5 Markets to Rice Exchange. Naturally, this better happens in our richest provinces (very fertile soil, gold mine, etc.) to maximize the additional profit.
(2) Don't bother upgrading the Market in any other provinces.
(3) Even with Metsuke, it is only more worthwhile to upgrade a Market to Rice Exchange if we play a shorter campaign and have a low number of provinces for the majority of the game. The loss of 1500 koku in early game makes a much bigger impact than the gain of 1500 koku in mid- or late-game.
(4) Never upgrade the market if you will finish the campaign in the next 20-25 turns. (It only makes more money 15-20 turns after its completion. The construction of Rice Exchange takes another 5 seasons.)
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