method of figuring out cost/advantages

I don't have a methodical approach to this; I just start off with a set of costs that "feels" about right and then tweak with playtesting. But there are a couple of guidelines that I try to keep in mind that can be helpful.

First is that when "building" or "purchasing" something, players are really incurring two costs: the resource cost of the item and the turn action to make the purchase. This actually gives you two handles you can use to tweak costs. For example, let's say that building A gives 1 resource, and building B gives 2 resources. As a starting point, B might be twice the cost of A. But what's the difference, then, between buying B or buying two of A? This is where the rest of your game's mechanics factor in. Perhaps you only get one action per turn, so B can be built in one turn whereas 2 A would take 2 turns. Or perhaps space on the board is limited, so B is a more efficient build. If either of those are the case, B perhaps should be a bit more expensive than double the cost of A. In contrast, if there aren't these restrictions, B could pay out a little more than double what A pays out, so that there's a distinction between the two.

Another important guideline is balancing the cost against the players' likely holdings, and making sure that a purchasable item is useful in the phase of the game in which players will actually be able to afford it. You don't want to have a building that would be useful in the early game be so expensive that players can't afford it until late in the game, when they won't need it anymore. In this sense, you need to balance costs cooperatively with balancing the resource production aspects of your game.

Other handles you can tweak include the scarcity of the items, or restrictions on the number or type that a player can acquire.

Unfortunately, then, I don't believe there is a generic answer to your question; there are just too many variables, so it depends on the other mechanics of the game to come up with a cost structure that is balanced. Happily, it seems you know the answer to your question: playtest, playtest, playtest!

-Jeff