The complex potential deal to transform GE’s 80-20 partnership with Vivendi (EPA: VIV) into a 49 percent stake in a programming merger with Comcast (NSDQ: CMCSA) may hinge on a $500 million gap in valuations. Keep in mind that Vivendi hasn’t formally announced its plans to shed its 20 percent stake in NBCU, the needed ingredient for the chain of events to take place. But Vivendi did tell GE unofficially over the summer that it wants to sell. Now WSJ is reporting that it all boils down to whether Vivendi can get book value for the stake—that would be about $6.3 billion as of June 30—but the two companies are about $500 million apart.

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One suggestion is that GE CEO Jeff Immelt’s talk of an IPO for NBCU and the internal detailing of that possibility has been to show Vivendi it can’t raise as much on the public market for its 20 percent as the $5.8 billion GE might be willing to pay to acquire the stake outright. Vivendi has until December to make its decision for this year; the same window opens every year until 2016. Meanwhile, detailed talks between GE and Comcast over their potential venture continue, with the outline remaining essentially the same: GE/NBCU buys the Vivendi stake, then combines the assets valued at about $30 billion with Comcast’s cable networks and its Fancast portal plus about $5-6 billion in cash from the cable operator. That would leave roughly $9 billion in debt that would be paid out of free cash flow. More as we know it.

Update: As a little lagniappe, former News Corp (NYSE: NWS). execs Peter Chernin and Peter Ligouri are reported to be involved on the Comcast side of the deal. The NYT, which reported Chernin’s involvement first, describes him as a paid consultant, while the LA Times says he’s acting more as a friend than a consultant, informally advising Comcast without being involved in negotiations. Former News Corp. COO Chernin and Comcast COO Steve Burke are friends; Burke would be the Comcast exec most responsible for making sure a deal works but would not head the venture. A source familiar with the situation told me last week that Chernin is not in line for an executive role should the deal go through. LAT also reports that Ligouri, the former chairman of Fox Entertainment who was left without a seat during the last round of Fox musical chairs, is a paid adviser helping Comcast value NBCU’s entertainment assets. Ligouri, who has deep studio and broadcast network experience, likely would stay on as part of the Comcast team.