There's two important considerations for the game economy.
1) When items are not able to be traded between players.
If the pack items aren't able to be traded/gifted/etc between players then don't worry about supply and demand. Just let people spend what they would like and be clear about what you can get from the pack. Mafia wars sets a great precedent here and because the supply really doesn't matter, you can just have the items be totally randomly dropped.
2) When items are able to be traded between players.
That's when things get tricky. In order to have a functioning game economy, items must not just flow into the game but they must also leave the game. So you need to make sure that there must be a way that the items players buy in the packs, if they match the functionality of game items, are either used up over time or they can be lost on death. The particular mechanics depend on your game but you absolutely have to balance items leaving vs items entering the game.
Then once you have the inflow and outflow of items balanced you can decide the specifics of whether you want the items to be totally random or balanced beforehand. In a large enough volume you'll get almost entirely the same result.
In this situation I personally would set it up such that a pack gives you similar items to a certain amount of repetitions of whatever you do in game to get the items. So you can kill 50 X-dragon that gives you a 50% chance of 10 of Y-dragonhide per kill then you can go kill 50 dragons in game to get 250 dragonhide, or you can buy a pack of 250 dragonhide. That way you're not completely alienating the players who prefer to "work" for their items.
That can be further balanced by simply allowing people to buy only the kills that could have happened, but nobody was online to kill the dragon(s). So if the dragons take 5 minutes to kill and respawn after 10 minutes, then you know that only 96 dragons can be killed per day. If there's only 50 dragons that are killed that day, then you can put 46 dragon kills up for sale.
I don't know of any games that have implemented any systems like this, simply because making items that can be traded between players purchasable tends to really piss off everybody who plays for "free".