[I see Alex has already addressed this point, but I'd already written this...]
Since the issue came up, I figured I'd take a specific look at Diversified Economy.
Diversified Economy -- $4, 2VP Consume -- Consume 3 different goods for 3 VP Produce -- Get one card for each type of good produced.
I called Diversified Economy one of the "Mini-sixes", in that it can provide a healthy stack of VPs and cards. Not all of the 6-Developments are created equal, though. Trade League typically generates a lot of cards and few points, while New Economy is the opposite. Diversified Economy is definitely 'card heavy.' You can drop this as your 3rd or 4th card and then earn 1-2 card every production phase. At that point, you'll still need a way to consume them, but early on you typically trade and then consume any spare goods. In the early game you are focused on your card economy. Compare Diversified Economy to Interstellar Bank. The bank gives you one card every development phase (for 2 cards). if you are getting two cards per production phase for a cost of four, that's about the same.
The difference is that you can (with a good engine) call Develop every turn, while Production is every other turn.
To balance that, Diversified economy has a huge mid- to endgame consumption power. Three goods for 3 VPs. Now, the goods must be different, but that usually means "You must have a genes or alien good." (Novelties and Rares aren't difficult to find). This power isn't much to write home about ... if you already have consumption powers. If you don't, it provides a huge punch. You also avoid buying other consumption powers, which means cheaper worlds. Production worlds without consumption powers are roughly 1 card cheaper, so you can expect 1-2 cards in savings (since you may have to bite the bullet).
Finally, D.E. lets you switch from building to produce/consume (and consunme x2) earlier, which may result in an extra consumption phase. Even if you don't use the consume power, the extra VP from that phase are directly attributable to this card. So, let's assume you get one extra cycle for 2 VPs, (since you traded and used other powers). So D.E. earns you 4 VPs (2 directly, 2 from one extra phase). You get 3 production bonuses (2, 3, 3) and get one world cheaper since you have 'spare' consumption powers. So for 4 cards (+D.E.) you earn ~9 cards in discounts/income and 4 VPs.
If you get an a bonus x2 consume, that number shoots way up.
If you become a production 'pioneer', then the fact that one of your worlds will be a windfall won't matter. For leeching, it will. Diversified Economy isn't a particularly good leech (although most of the 6-Devs aren't, either).
D.E. doesn't combine well with many of the Six-devs, but not all of them combine well enough. It still works with with Free Trade Association or Mining League, although they'll wind up fighting over goods. It's happy with Merchant Guild, and useful with Pan Galactic League or Alien Tech Institute (you'll be able to use the consume power, and neither of those helps in that regard). Most of the other Six-Devs are fairly neutral. (D.E. is +2 VP with New Economy, but you'll probably not get as many VP from your worlds).
Sparta, in particular, often gets a smattering of good and not enough consume powers, which means that even a late D.E. may pay off 3 cards and 5 points (although sometimes you just get the cards).
Overall, even a reasonably late play of Diversified economy will usually net you 3 cards and 3 additional VP, making this a pretty cheap VP boost. And this can hit early, providing a strategy-in-a-can that drives your whole tableau.