Internet ad firm 24/7 Real Media is on the auction block and has already been talking to Martin Sorrell's WPP Group, the world's second-largest ad giant, among other potential suitors.

New York-based 24/7 Real Media has been flirting with Britain's WPP for several months, although price is proving to be a sticking point, said sources familiar with the talks.

Analysts said 24/7 Real Media could fetch upwards of $600 million, or at least three times the company's annual revenue. The actual asking price could be a lot higher, based on recent deals.

Google coughed up an eye-popping $3.1 billion in cash for Internet ad firm DoubleClick, which generated an estimated $300 million in revenue.

Similar to DoubleClick, 24/7 Real Media has software that allows Web publishers and advertisers to serve up and track ads. It also has a network of sites that run ads 24/7 Real Media brokers on their behalf.

WPP, home to Madison Avenue agencies JWT and Ogilvy & Mather, wants to be considered the leader among the traditional ad agency holding companies in the fast-growing digital marketing field. Although WPP chief Sorrell has been investing in smaller interactive and mobile marketing companies, he has yet to make a sizable acquisition in the space.

Meanwhile, French rival Publicis Groupe just bought Internet ad firm Digitas for $1.3 billion.

"Sorrell wants to look like he has a digital strategy," said one source familiar with the talks.

WPP and 24/7 Real Media have not commented so far.