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I'm in the process of building a game in which virtual items can be obtained either by in-game play (defeating enemies, gaining levels), or by purchasing "packs" via microtransactions.

Looking at an existing example like Duels.com's item packs, it looks like a lot of thought went into their implementation, including:

  • Setting clear player expectations as to what can be obtained in the pack
  • Limiting pack supply to increase demand and control inflation

Are there other considerations that should be taken into account? For example, should the contents of the packs be pre-generated to guarantee the advertised drop rates, or is each drop rate just a random chance, and you could end up with higher or lower supply?

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4 Answers

up vote 5 down vote accepted

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".

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I been playing LOTRO for 3 months now. There are tons of items in that game. However, equips (weapons, armor) and other very cool items are bound to your character, so like Kort Pleco said before, there is no trading for those items. So, the game is very balanced, because if I have a superb item, there is no way that item could be sold when I dont need it anymore, and sold again, and again... You get a nice item (crafting it, looting it, buying it from the market) but is just for you, and when you dont need it anymore, the only thing you can do with is destroy it. The binding system isnt new, but I think its very useful.

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To add to Kort Pleco's wonderful answer:

  • Are items permanent or consumable (or both)? For example, many free-to-play games offer microtransaction-purchasable items that do something like "double your experience for the next 24 hours". If players are going to be getting items but then using them, inflation and availability are less of an issue.

  • Bulk rates. Do players get extra stuff for buying a lot of item packs at once? For example, maybe buying a single pack has a 10% chance of holding a "super-rare" item; maybe if you buy a box of 10 packs, you guarantee the player at least one super-rare.

  • Generally you don't have to worry about whether drop rates / random distribution will conform exactly to expectations or not; the laws of probability dictate that the more times you do something random, the closer your overall distribution conforms to the expected value. So if you're afraid of randomly generating too many of Item X and not enough of Item Y, enough item drops will fix that over time.

  • HOWEVER: absolutely do some statistical tests on your item generation code to make sure you are actually generating things according to the probability curve you thought. A software bug here will absolutely be noticed. (I actually did see some production code in a card game once where the shuffling algorithm was not done correctly, so certain cards tended to get shuffled to the top more often than others, so I'm not kidding about this.)

  • If players can trade/gift items to one another, there are a LOT of additional considerations. Are you okay with players trading their stuff for cash, i.e. Player 1 sells their stuff through Paypal to Player 2, and all you see on your server is an "item gift"? If not, what steps can you take to stop it, and how much time are you willing to spend cracking down on this? If so, what will you do when a player inevitably rips off or scams another player for money or items - will you actually reverse the trade, or will you leave your players twisting in the wind... and if you DO reverse the trade in the case of scams, keep in mind that'll cause scamming to proliferate because your players will think of you as a safety net. Or do you force all transactions to happen through your servers, including cash - and if so, are there additional legal issues since you're now effectively acting as a bank or payment processing service in addition to a game service (let alone any potential charges of "gambling" since players can spend or make money on your game that has random elements to it)?

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I'd suggest you to completely separate "things that are created in-game" from "things you can buy from outside the game".

If you do, you raise the value of both the items bought (because they can't be obtained just farming) and the value of the farmable items (because they can't just be bought); this way you don't alienate any player.

Then, regardless of how these items are obtained, they can be traded (of course you have to pay attention to all the economy problems).

You are right about the importance of player expectations for the pack contents, about what I suggest:

  • there might be variations on quantity, but small variations (such as 3-5 of this, and 6-9 of that)
  • there shouldn't be anything very rare, or some people will buy a lot of packs without getting it, become angry, and not buy any more packs again
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