Tapjoy, which drives downloads for mobile developers by cross-promoting apps across a network of titles, launched a $5 million fund to port iOS apps to Android.

Basically, for certain types of games (e.g. freemium ones that monetize with virtual goods), Tapjoy will finance adapting them so they can work on Android devices. The assistance could include just financing, or it could involve Tapjoy actually doing the port in-house with their own developers, or outsourcing it to other companies. Porting a title can cost anywhere from $50,000 to $100,000, depending on what language the app is originally written in.

In return, the company’s wants the ability to distribute and monetize these titles through offer walls once they launch on Android. In some cases, Tapjoy will give developers free credits that will cover the cost of downloads.

For background, Tapjoy is the biggest player in incentivized installs, where developers pay every time their app is downloaded through an offer wall in another game. Game developers often host offer walls so that players who can’t or aren’t willing to pay for virtual currency in a game can get virtual rewards for free if they download other apps.

The fund’s launch comes at a sensitive time for Tapjoy after Apple began cracking down on such offer walls two months ago, arguing that they are used to manipulate chart rankings. Since then, many of Tapjoy’s biggest partners have pulled their offer walls out in order to comply with Apple’s new policies and the company’s network on iOS has likely shrunk dramatically since then.

However, Android is picking up as the platform is seeing north of 400,000 device activations a day. Tapjoy’s Chris Akhavan, who heads the company’s publisher development team, said that in December, Tapjoy was driving “tens of thousands” of downloads a day, but now it’s doing “hundreds of thousands” of downloads.

He added that Google’s in-app billing system is turning out to be more comparable to Apple’s in-app purchases system than the company thought.

“In some cases, top developers are telling us that the share of revenue coming from in-app billing [versus advertising, etc.] on Android is higher than it is on iOS,” he said.

He added that because Android Market’s rankings take much more than download rates into account, it’s harder to raise the criticism that Tapjoy’s incentivized installs are gaming store rankings. Still, the company’s trying to offer other products like cost-per-engagement (or paying every time a user reaches a certain action in the game like finishing a tutorial or leveling up to a certain point) that counter skepticism about their model.