I had read somewhere (prolly here at GD) that all people are not super rational (paying more for less when getting bulk game currency, acting as collector) , hope I can find a sustainable mechanism not to exploit weaknesses or overcater whales.
I personally like to think of it of creating something that is highly valued by customers, no matter what its true worth is.
Lets take personal computers for example. For some reason, Apple computers usually demand a premium over a similarly specced no-name PC, and even then sell like hotcakes while the no-name PC waits for the Sale season in the shop shelf.
IF you look at the raw specs, there might be one or two clever solutions from Apple, but nothing groundbreaking. If you look at the build and materials used, Apple usually is rather on top of the game, still, the asian no-name companies learn fast, and apple look-and-feel-alikes are a dime in a dozen nowadays. If you look at the OS and Software, things tend to get even more "religion" and less tangible advantage, even though the difference is bigger here.
Apple, of course, has a recognised Brand by now and is master of advertising and selling stuff.
The combination of all these small things adds up to a product people really want to posses if they need it or not, rather than have to buy because, you know, gotta own a notebook nowadays.
From what I have seen is that by applying similar strategies in online economies, you can design a virtual good people want to own, even though it gives them little ingame advantage.
It could be a different playstyle, a better looking vehicle/hat/mount/whatever, faster leveling, a recognizable Brand/Person/Name/Whatever as opposed to a no-name one, the urge to finish a virtual collection (maybe by additionally giving achievments and additional goodies for completing the collection), that drives people to invest money in virtual goods without ingame bonus.
Lets say, you have a game where you fight cowboys as an Indian. it is free to play, and you get a no-name Indian. Good looking, but rather bland chap, with his single feather, and rather dull looking equipment. Does to job fine.
Or you could buy an alternative character to play as, Crazy horse. It is a historical figure many know about, and many might want to play as rather than average joe Indian warrior. Then you get this amazing 3D model, with the Feather crown, sparkling looking clothing and amazing weaponry. A true chief amongst Indians. Perfect to show off in front of these free loaders.
Make him play a little different (we know Crazy Horse went a little mental in battle, but seemed to be a very talented warrior, so he could be a better warrior, but somehow harder to "control"), but balanced against average joe indian, and some people might just pick him up for that, even though average joe does the job just fine.