I'm just learning the game, so some of my comments may seem ill-informed. But it seems that strategies fall under two categories, a commodity-based strategy and a building-based strategy.
Commodities are simple enough. Produce a lucrative commodity (and take on roles that will increase production or profitability), then "cash out" for victory points at a high rate in foreign markets. This is what I call a "constructive" strategy.
Another strategy is to invest in the local economy in the form of buildings. At first blush, this seems less lucrative than selling commodities. But many winning players prefer this strategy, perhaps because it gives them some "veto" power over other players' shipments. This is what I call an "obstructive" strategy.
In a game such as backgammon, I would use a "constructive" (running) strategy if I got high rolls early in the game. With low rolls, I would use an "obstructive" strategy of building hurdles to make opponents "waste" rolls. Could an analogous dynamic support the acquisition of buildings in Puerto Rico?
It also seems that with buildings that there are hidden "synergies" or feedback effects. For instance, the "list price" may be five doubloons for a building with three victory points. But under some circumstances, the building "kicks back" two doubloons, which may be used to buy another building (and more victory points. Or it may "kick back" a colonist that may be used to upgrade another building. Meaning that the five doubloon building actually costs "less" than its "list price."
Could it be that buying buildings is a highly "constructive" strategy, but only if you buy several of them in tandem?