I never thought I'd play a fantasy-adventure game in which I'm both an epic hero and a landlord, but thanks to Peter Molyneux that's exactly what I became in Fable II. Early on in the game I bought every house I could afford, and shortly thereafter the rent started rolling in. Unusually, the rent payments from my tenants were based on real time, not played time. If I put the game down for a week and came back to it, I'd have tons of rental income waiting for me.
Fable II had a surprising amount of economic gameplay. You could work a variety of menial jobs like wood-splitting; buy goods in one town and sell them in another; take advantage of temporary sales to load up on cheap goods; purchase businesses and receive both a cut of the sales and discounts when you shopped there; and you could furnish your properties with better items to raise their rental value.
Depending on your other activities, you could also make a given town more or less prosperous. If you had a lot of investments in a particular town, making that town safer would increase business and raise your income. I suppose you could also make a town less prosperous for purposes of depressing prices there so you could reliably buy cheap goods and sell them in happier places.
Despite all this, Fable II's economic gameplay was crap. It was too easy to get too much money too fast, and after that you just never cared about money anymore. You might keep buying houses for the sake of achievements, but you didn't really need the income.
I'm having a similar experience in Assassin's Creed II. There's a small town that serves as your stronghold and you can pay for a variety of upgrades to the town which improve your income. You also get discounts from businesses you help fund. I was pretty jazzed about this because it's an alternative progression system: I'm gradually improving my character but I'm also gradually improving my town, both of which have visual and gameplay ramifications.
Unfortunately, I maxed out the town upgrades very, very quickly. Money flows at you steadily in Assassin's Creed II and within just a couple of play sessions I had finished all of the upgrades. I'm still making small improvements to the town's value because the various items you collect make the town more prosperous - although I'm not sure why the town makes more money when I buy a sword - but what I thought would be an enjoyable, ongoing sideline to the main storyline turned out to be a very brief and shallow experience.
In both cases, these games have problems with basic economic gameplay: faucets and sinks. Faucets are how money enters a game and sinks are how it leaves the game. If the flow from faucets outweighs the drain from sinks, the player has too much money and it loses its perceived value. Shopping isn't much fun when you can buy everything because the joy of shopping is the trade-off between what you want and what you can afford.
MMOGs have experimented a lot with economic gameplay. One thing they've done very well is expanding the sinks. Consumable items are excellent sinks: health potions and the like, since you keep using them and buying more as replacements. Damage and wear to armor and weapons is another sink since you're basically paying rent to keep items you already own.
However, there's a third element MMOGs have done very well that single-player games haven't: markets.