FXX's deal last week to show "The Simpsons" in syndication set a record for the market in TV reruns with a cost in the ballpark of $750 million.

On a per-episode basis, however, the latest syndication payday has some precedent. TBS shelled out more than $1 million per episode for "Seinfeld" in 1998, and Spike paid an estimated $1.9 million per episode for "CSI: NY" in 2004. Even as cable channels including FXX increase spending on original series, moreover, their desire for top-notch broadcast reruns hasn't waned. In recent years, TBS paid an estimated $1.5 million per episode for "The Big Bang Theory" and $1.7 million for "2 Broke Girls," while USA ponied up $1.4 million an episode for "Modern Family."

How do these deals still make sense? They're about the ratings, of course, but not only that.

"These shows are still valuable because they are a known commodity," said Bill Carroll, VP-director of programming, Katz Media Group, a media buying firm. "Networks don't need to establish them." That means cable channels can piggyback on all the broadcast networks' promotions for the series that have come before, as well as a familiarity among viewers that can go back years. Their own original shows have to scramble for tryouts with viewers.

Some series' syndicated runs also attract a younger and more-male skewing audience -- a demographic that TV networks have had difficulty finding -- than they got in their first lives on broadcast, according to Mr. Carroll, who follows the syndication marketplace for Katz. That's partly because of the time slots involved: Young men are more readily available before or after prime-time, he said.

For FXX, a fledgling channel that arrived in about 72 million households in September, "The Simpsons" is a way to establish itself. If "The Simpsons" helps bring FXX into another 20 million or so homes, the deal becomes profitable even before advertising on "The Simpsons" itself is factored in, said Brad Adgate, senior VP-director of research at Horizon Media.

On the ad side, "The Simpsons" doesn't necessarily need massive viewership on FXX, Mr. Adgate said, because its core 18-to-34 demographic is one marketers covet. Now in its 25th season, episodes from the 90's might seem dated to some, but there's a whole generation that will be seeing them for the first time.

Cable channels once had to wait many years for their turn with a big broadcast hit. Syndication rights to sitcoms were originally sold first to broadcast affiliates, and then sold to cable in a subsequent cycle that could come as many as seven years later. TBS struck a deal for "Friends" in 1996, but the series didn't actually appear on the channel until 2001. That window has closed significantly over the years, with studios recently selling sitcom reruns to broadcast affiliates and cable concurrently, Mr. Carroll said.

If cable's place at the table has improved over the years, however, digital newcomers are now crowding their way in too. A big part of the appeal of "The Simpsons" is its previous absence not only from cable but streaming platforms such as Netflix or Hulu, Mr. Carroll said. The FXX deal includes full digital rights, allowing the network to make the cartoon available on its upcoming application FXNow.

"The value is based on lack of availability," he said.

That kind of scarcity is becoming harder to come by as studios increasingly make past seasons of series available digitally. When TBS picked up "Friends" and "Seinfeld" in the 90s, the shows could only be watched on local affiliates, but now "Seinfeld" reruns can be found on platforms like Crackle and Hulu as well.

The value of reruns on cable greatly depends on how deals are structured and how well networks can capitalize. Typically, studios sell the show to broadcast and cable to air in different time slots, usually in prime-time on cable and in slots right before or after prime-time on broadcast. Some deals also give cable limited access -- one or two episodes per week -- in the season before full daily syndication begins.

"The Big Bang Theory" doesn't run in the same time slot on broadcast as it does on cable, allowing viewership to be complimentary -- or at least not cannibalistic.

As more shows become available on multiple platforms, cable networks are trying to lure viewers by scheduling reruns around themes and topics, a programming strategy called "stunting." FXX intends to stack episodes of "The Simpsons" in themed nights with focuses such as certain characters, guest stars and holidays. USA Network has been doing something similar with its reruns of "Modern Family," with nights titled "Say what, Gloria?" and "What's the plan Phil?"

For advertisers, at least, the appeal of reruns is as simple as the ratings, said Sean Galligan, group director at media agency The Media Kitchen, with reruns of many broadcast sitcoms performing better for cable channels than their own originals.

TBS' particular success with "The Big Bang Theory" has breathed a new life into cable's desire for broadcast repeats. The sitcom currently averages 2.4 million total viewers and 1.2 million viewers in the all-important 18-to-49 demo, according to Nielsen. Last week, "The Big Bang Theory" on TBS accounted for 19 of basic cable's top 20 sitcom telecasts.

But if ad buyers find networks' prices too high, they have another option: the ad time controlled by the syndicators themselves. Media buyers tried that when USA sought higher ad rates in the upfront for "Modern Family" than advertisers previously paid for reruns such as "NCIS" in the same time slots.

But inventory is limited. While each deal is different, typically for a sitcom, the cable network sells about five to six minutes of commercial time and the syndicator sells about two minutes, or four 30-second spots, and promotional announcements, Mr. Carroll said.

USA and parent NBC Universal have been aggressive in marketing "Modern Family" like an original. Known for its blue-sky dramas, USA is using the sitcom as a launch pad for its own forthcoming original comedies.

But since the series debuted in September, it has averaged 1.2 million total viewers, nearly half the viewers the network was averaging at the same time last year. NBC Universal has made up for ratings shortfalls by giving advertisers so-called "make goods" in inventory on NBC in shows like "The Tonight Show with Jay Leno" and "Late Night with Jimmy Fallon," according to a media buyer.

But for USA, "Modern Family" serves several purposes, not just the chase for ratings. According to the network, "Modern Family's" audience is 11 years younger on average, more affluent and better educated than the average USA viewer. And week-to-week ratings are markedly improving.

Networks are already looking forward to the next batch of shows entering syndication in the next few years, although nothing on the scale of "The Simpsons" is pending. In the next two years, "Mike & Molly," "2 Broke Girls" and "Raising Hope" will bow on cable and "New Girl," "Suburgatory," "Mindy Project," "Last Man Standing" and "The Neighbors" may be shopped in the market. None of these have yet to prove themselves a hit akin to "The Simpsons" or "The Big Bang Theory."