Anyone had their hand in developing "freemium" or "pay-for-extras" games?

Hello Warriors.

I'm not sure how many gamers are here on the WF, but I'm sure, statistically, there have to be a few...

I've played Entropia Universe, which is a "freemium" 3D role-playing game. You can play the game for free, and even earn real money by playing, with or without investing. Obviously, tons of effort went in to balancing their potentials for payouts against their potentials for pay ins, which I doubt very seriously they had balanced when they released the game. You can kill monsters for "random" treasures, you can mine for minerals, build stuff, and even own land or stock in the universe! The idea here is to make it so the "average" player loses money, but in a way that is fun enough to encourage the player to deposit more funds, to keep playing. Some make it big, some lose every penny they put in, but all in all, they seem to be profitable. Most actions in the game are compared to gambling, you can buy X amount of bullets, go kill Y amount of monsters, and hope for Z amount of treasure.

There's also quite a few browser games out there like Anno1777, where you can start with no investment, and potentially earn real money. I don't have a whole lot of experience with sites like these, but I know there are quite a few out there.

So my question is, have any Warriors been involved in something like this? I have years of experience with table top, pen and paper role playing, and I think a typical "d20" system could EASILY be adapted for "freemium" or "pay-for-extras" online play. Certain rulesets are copyright, so I couldn't make money hosting a "world" that uses their rules, such as Dungeons & Dragons, but there are other rulesets out there that basically offer total freedom to the end user. It wouldn't even be beyond my skills to develop my own ruleset.

The main point of my post, is to find any insights you folks may have in finding that balance, to where, for the player (customer) the risk of investing isn't greater than the potential for reward, reward being gameplay, and possible withdrawals. In Entropia Universe, in game currency has a static 10:1 ratio, so you can buy 200 credits for $20, or sell 200 credits for $20. You can take 10 credits worth of bullets and kill a monster, and basically "gamble" on whether that monster drops nothing, a few things (break even) or tons of stuff (jackpot) to make a profit. The odds are carefully calculated, monitored, and constantly balanced to make sure that more money comes in, than goes out, on average.

Anyone have experience in this field?