When it comes to in-app micro-transactions on iPhone and iPod touch, one of the most interesting areas are the cross-platform games that include iDevices with other platforms such as web and social platforms.

One such is massively multiplayer online card battler Urban Rivals.

Originally an i-mode mobile game, it added a web interface, and a paid-for iPhone app is due to be released soon.

You can play the same game from all three clients as they link into the same server infrastructure.

What's significant in terms of the game's business model is because the web game is free to play, you buy packs of character cards to build up your experience and make quicker progress.

These are purchased either by earning Clintz, the in-game currency, used to by cards from other players, or more quickly using real cash, which is converted into Credits that can be used to get packs of new cards from the shop. Players also earn Credits through tournaments.

As previously discussed with a-steroids, developers of the Underworld: SweetDeal game, Apple doesn't allow apps to use their own in-game currencies.

This is the case even in terms of users buying their in-game credits on the web and want to spend them using the iPhone app.

Complex stuff for sure, so we caught up with Nick Witcher, Urban Rivals' US and UK marketing manager to find out how publisher Boostr was dealing with the issue.

Pocket Gamer: How will you be using micro-transactions?

Nick Witcher: Urban Rivals is a trading card game, so we'll be selling additional cards.

How do the restrictions on using in-games currencies for the iPhone game affect you as players can buy Clintz via the web?

We got around the issue by selling packs of cards as in-app transactions rather than in-game credits. Of course, you can still win free credits while playing the game via the iPhone app, and use them to get pack of cards, but you can't purchase credits within the app.

How will you be approaching micro-transactions in terms of minimum 99c price point?

Our case is a bit different because we are already using a micro-transaction business model on the web and know that we can charge more than 99c.

On the web, our price ranges from 99c to $40 depending on how many card packs you want. On iPhone however, we'll only have two prices 99c and $9.99.

What will gamers get for those two options?

You either purchase one pack of three character cards for 99 or 11 packs for $9.99.

There seems to be some backlash from gamers about in-app transactions. How do you think developers can overcome this?

Because we're a trading card game, micro-transactions have never been an issue with Urban Rivals.

One thing that is important however is that on the web Urban Rivals is free and players are getting a lot for this ‘very reasonable price’. For example, you can get a lot of the cards just from playing the game and entering the tournaments and there are no limits to what you can do for free - there's no per day cap on fights, or premium-only features.

So when a player decides to spend some money on cards, it's not because they're not having a decent game. It's because they already have a very good game for free but spending a bit more makes it more fun.

I think that's very different to purchasing extra levels for a game you’ve already spent money on but thought was too short.

How significant do you think it would be for Apple to enable in-app transactions for free games?

Based on our experience from the web, it might be huge.

Thanks to Nick for his time.

You can register for the online version of Urban Rivals at the official website.

The iPhone app will be out soon priced at 99c, €0.79 or 59p. There will also be a free version.