Economic Victory?

Since there probably is not any way to win with any of the other approaches without successful economy, typically having an economic victory path is redundant. If your economy is truly dominant, you probably can buy your way to any of the other paths. In fact, that's often the way I try to win these sorts of games. :)

That said, there are two approaches to economic dominance: First, if you just have more income than anybody else, by an amount more than they could catch up in two rounds. You could represent this as having two or three consecutive rounds in which you earn more than everyone else in all of the resources. Of course, this would mean you have to track who earns the most in each one.

If the way you get income does not fluctuate quickly, and there are four or more different resources, then it is probably enough, as victory condition, to say that one round of making the most in every resource is an economic victory. This doesn't work if the income is based on something fleeting, like cards played; it only works if it is based on something ongoing, like owning mines. (Is this point clear why? With a large turn-to-turn deviation, just managing to put together one round where you get highest in every resource doesn't represent 'dominance.' It has to be something you're going to be repeating, or nearly so, for successive turns.)

The second form of economic dominance is if you are able to completely control a key resource, such that you can starve the progress of the others. If there are mines, farms, or whatever for each resources, then a player who controls them all can declare a monopoly and either block other players from getting that resource, or significantly increase the cost for it (and, of course, he gets it free himself). The victory condition could be to have any two monopolies.

Of course, that gets back to my first point, which is that such a victory condition would probably be superfluous. Someone with two monopolies would probably be able to squeeze all the other players' progress while just buying the technology and/or military to achieve either of those victory conditions.

I don't know if either of these makes sense in your games, but I hope it provides some inspiration. Good luck!