The fact is that Palm's value was decreasing fast despite new products like the Pre3, Veer and Touchpad, making it simply impossible to sell at the same price. $500 million would have made it more likely to sell according to reports, but HP wasn't allegedly willing to go that low. At one point, Facebook's CEO Mark Zuckenberg was reportedly sitting in at a meeting where HP was being literally laughed at for asking $1.2 billion in exchange for Palm.
Facebook wasn't the only one turning down HP; Samsung, Amazon, Intel and others are all mentioned in the report as companies that might have been interested in an acquisition, but not for the price HP was asking.