Talk about an Eye-popping number: CBS has been seeking half a million dollars from advertisers who want to tuck a 30-second commercial alongside the March 31 series finale of “How I Met Your Mother,” according to people familiar with the negotiations.

Not everyone will pay that sum. Advertisers who bought the program during last year’s “upfront” market, when networks seek to sell the bulk of their ad inventory for the fall season, were able to secure ad berths in the finale for between $300,000 and $350,000, according to these people.

Marketers forking over top dollar to be in the “HIMYM” finale will be paying a significant premium for the show compared to its normal run in the season. According to a recent Variety survey of primetime ad prices for the 2013-2014 season, the average cost of a 30-second spot in the show was $169,364. The series stars Josh Radnor, Jason Segel, Cobie Smulders, Neil Patrick Harris, Alyson Hannigan, and Cristin Milioti.

“People are paying for the expectation of a great audience delivery, and they want to be part of something everyone talks about the next day,” said Dean Kaplan, exec veep of sales strategy, planning and administration at the network. CBS has only a “handful” of ad slots remaining in the series finale of the show, said Chris Simon, exec veep of sales at CBS.

CBS in December said it would run an expanded version of the last episode of the show which has run for 208 episodes over nine seasons. Originally envisioned as airing in May, toward the end of the season, the finale will now run in March. CBS plans to use the “HIMYM” finale to launch “Friends With Better Lives,” a half-hour sitcom starring Kevin Connolly, Majandra Delfino, James Van Der Beek, Zoe Lister-Jones, Brooklyn Decker and Rick Donald as six friends at different stages of their romantic development.

“Friends” will air at 9 p.m., March 31, after the hour-long “HIMYM” denouement starts at 8 p.m. In following weeks, “Friends With Better Lives” will air at 8:30 p.m. Mondays.

The network is also banking on the success of “HIMYM” to help it develop a spin-off of sorts, “How I Met Your Dad.”  Greta Gerwig is set to star as a woman on the verge of breaking up with her husband after less than a year of marriage.

While the price for the last episode of “HIMYM” is high, it isn’t the most expensive series finale that has appeared on television. In 2010, for instance, ABC sought between $850,000 and $900,000 for a 30-second spot on the last episode of“Lost, “ while Fox asked for $650,000 for the same amount of inventory in the series finale of “24.” Half a minute of pitch time in NBC’s 1998 series finale of “Seinfeld” cost between $1.4 million and $1.8 million. A 30-second ad berth in the last episode of “Friends, which ran on NBC in 2004, went for between $1.5 million and $2.3 million, while a 30-second slot in CBS’ final episode of “Everybody Loves Raymond” cost around $1.3 million.

In a sign of how prices have risen, a 30-second spot in the 1983 series finale of “M*A*S*H,” one of the most-watched TV programs of all time and broadcast on CBS, cost just $450,000.

Advertisers are clearly banking on a massive audience. At $500,000, “How I Met Your Mother” would cost sponsors more than two of TV’s highest-rated programs: ESPN’s “Monday Night Football,” where the average cost of a 30-second ad is said to be $408,000, or a package of ads on AMC’s “The Walking Dead,” which advertisers pay around $326,00 for the privilege of using.

To be certain, not every advertiser buying “HIMYM” late in the process is bound to pay the full $500,000 figure. The numbers often represent a first offer, with buyers typically responding with a lower figure. And negotiations continue.