Hewlett-Packard shares had fallen $1.23, to $30.16, as of 1:41 p.m. on the New York Stock Exchange. They earlier had jumped as much as 8.3%. Up until this week, the shares had declined 25% during the year.

Autonomy's U.S.-traded shares rallied 51%, to $38.50, for their biggest intraday gain in six years, Bloomberg said. The shares had declined 8.3%, to 1,429 pence in London, where markets closed before news of the takeover talks.

Hewlett-Packard would be paying a high multiple of sales for Autonomy if the $10 billion price was accurate, Lamba said. Autonomy may report $1.05 billion in sales this year, according to the average estimate of analysts surveyed by Bloomberg, which would value it at about 10 times sales in a deal.

"That's a very high multiple for a software stock," he said. Software companies commonly sell in mergers and acquisitions for five to seven times sales, Lamba said.