Thanks in large part to the popularity of the freemium business model amongst mobile app developers, mobile ad platform Tapjoy has built an advertising network that reaches some 70m active users each month.

Now it’s hoping to really cash in on that audience by opening up its ad buying marketplace to all advertisers.

The Tapjoy Mobile Value Exchange allows advertisers to bid on ad inventory on a CPC basis, with a minimum bid of 10 cents. The company’s press release explains:

Whether an advertiser is looking for application installs, video views, registrations, subscriptions, form submissions, quote requests, surveys or any other type of CPA ad engagements, Tapjoy can help push their message out to millions of users at whatever price they set. Advertisers can begin running ad campaigns…with no minimum spend and can manage and optimize their bids in real-time. Dialing volume levels up or down is as simple and instant as adjusting bid levels in the right direction.

According to Tapjoy’s President, Mihir Shah, “The freemium market for mobile app developers has reached a very exciting point of scale. Our industry has experienced global adoption of the model with a broad and engaged set of users. Now is the time to provide our users with increasingly relevant and targeted advertisers that work for specific affinity groups. It’s still early, but we see significant headroom in encouraging users to self-select those advertisers that appeal to them.”

With mobile advertising revenue growing rapidly, it’s not surprising to see companies moving toward self-serve models. After all, to truly facilitate large-scale, efficient buying and selling of ad inventory, it’s all but necessary.

But doing this successfully may not be easy for everybody. Tapjoy, which is the latest incarnation of Offerpal Media, isn’t just an ad network; it’s in the business of incentivizing app installs, and some of its advertisers are app developers themselves. That wasn’t something Apple liked, so the company has had to adapt more than a few times.

At the end of the day, as we’ve seen on the web, quality counts. There’s plenty of opportunity in the mobile ad space, and we’ll see which companies and which models thrive.