Econ represents more labor per point than wealth, though, i'm pretty sure; its more expensive to build a watermill and have a dozen people work it than it is to assign a few hundred to more primitive mills, but it does save manhours/labor, because it only requires, say, 2 dozen people to produce that wealth, because wealth producers like miners are relatively elite trained professionals. So we trade ~3 dozen workers between the watermill and in mines/snail cultivation/etc for a few hundred in primitive mills
Well, let's break it down. I'll work with Mains, because Secondaries often have costs/benefits which are rounded off and ignored.
[Main] Build Mills -3 Wealth, -1 Art, +3 Econ
We spend 3 Wealth of fancy stuff (probably metal bearings, carefully shaped wood, etc.) and 1 Art of artisan labor to save 3 Econ worth of labor.
Suppose you generate that wealth and art via a Vineyard. If you spend a 'main action' of the government's directable labor force on preparing a Vineyard, and 1 Econ of labor to run it, you can get 1 Wealth of wine that you can trade for the fancy stuff you need, and attract 1 Art of artisan labor. If you double that, you'll get close to the cost of building a mill (+1 Art, -1 Wealth, but you can convert between those without too much effort.)
So to build a mill, you have to spend roughly three 'main actions' of government work force to do the logistics, etc. on building two vineyards and a mill, and devote two Econ of labor to running the vineyards, for a net profit of freeing up one Econ of labor.
I think three main actions of government work force is a lot more labor than 1 Econ (which can be created by half a secondary action). It's not possible to do an exact equivalency of course, so this is an estimate, not certainty.