The unnamed sources said the BlackBerry-device maker has been in talks with Millennial Media, an independent mobile ad network based in Baltimore, but that the talks have stalled over disagreements regarding the value of the deal. Millennial is reportedly asking for $400 to $500 million based on the recent prices that AdMob and Quattro were able to score ($750 million and an estimated $270 million, respectively). Both companies declined to comment, WSJ says.

Of course, there are other free agents in the space that RIM could investigate buying, including JumpTap and Greystripe, which have been very vocal about how much their networks are growing in lieu of the recent purchases by Google and Apple, which have successfully drawn attention and more advertising dollars to mobile.

RIM’s user base can be looked at as a valuable demographic for advertisers, given that a majority of its install base are business users. However, those users can be already targeted today by using existing mobile ad networks, so presumably, RIM would be able to bring something additional to the table if it owned its own network. Likewise, RIM would be able to skim some of the profits off for itself, which both Apple and Google are doing through its ad networks.