Strategies on upgrading buildings and plantations

[jinif]

I was curious how other people handle the upgrading of revenue generation buildings.
Especially the sugar/tobacco etc plantations which are in abundance in the New World. With upgrades costing around 2,000, it seems that it will take many turns for the investment to turn a profit, so is it better to use the 2,000 gold buying something else, like ships or units?
The same questions applies for mines and factories.
Are certain building, (Like ironworks, or gold mines) considered "must upgrade" each time a new version is availible?

Also, how does the "yield" on plantations affect your decision to upgrade?

[Fiddler]

Check the price on the different goods that you can upgrade, and see how long it would take for the investment to start making a profit. How stable your relations with other nations are is also very important for this decision, since being blockaded, say by France, will drain any profits that you would have received from the increased productivity.
Tackling more than one enemy with no backup or ready forces usually results in blockaded shipping lines and a flatlining economy for me whenever I play as a trade heavy nation, such as the United Provinces or Britain.