Yes I believe you are correct, about $7B would be needed by Liberty to buyout the remaining SIRI/XM.
One additional favorable feature about SIRI/XM is that its becoming a bigger and bigger Cash Cow.
And in this low interest rate environment, this cash cow machine offers a very attractive feature that any acquiring company could very quickly benefit from.
IMO favorable 02/15/11 earnings combined with a favorable 03/07/11 outcome with Liberty and Siri sees north of $3 in the short term.