It has been six years since baseball author Jonah Keri wrote “The Extra 2 Percent” chronicling how the Tampa Bay Rays became a repeat playoff team with baseball’s lowest payroll.

As the book’s title implies, the Rays and general manager Andrew Friedman were finding hidden value others overlooked. It was similar to the Oakland Athletics in the “Moneyball” book from 2003, only by 2011 so many teams employed statistical numbers-crunching that any advantage gained was more modest — the hypothetical “Extra 2 Percent.”

During a conversation last week with Keri, who now writes for Sports Illustrated and CBS Sports, I asked whether the value-finding premise of his book still applies.

After all, baseball since 2011 has seen significant changes — including television revenue that has driven team payrolls sky-high, not to mention two additional wild-card spots that also encourage higher spending as more clubs seek the baseline .500-record that assures playoff contention.

It is hard not to notice that, as MLB spending soars, the Rays have made the playoffs just once since the year Keri’s book was published and missed badly the past three seasons. The Mariners have not advanced in 16 years, and recent year-end payroll figures from MLB show them still in the game’s middle spending tier at No. 12 despite upping payroll by about $60 million since 2011.

Again, the flood of TV money makes it all relative. The Mariners spent $153 million last season but are no more competitive in terms of payroll than when also at No. 12 following former general manager Jack Zduriencik’s first season in 2009.

Anybody following the team this decade knows its biggest weakness was poor drafting and player development, especially compared with recent American League playoff squads such as Houston and Kansas City. Problem is, the Mariners also are not compensating for their developmental shortcomings by outspending playoff teams like Boston and Texas.

Considering the Mariners have missed the playoffs by a game or two twice within the past three seasons, they could use an “Extra 2 Percent” lift in value from the moves this winter by GM Jerry Dipoto.

The question is whether it is still possible to be even that much smarter than rivals.

“We’re down to like 0.2 percent,” Keri told me. “I don’t even know if 2 percent is an attainable goal anymore.”

Again, just as more teams spend barrels of additional cash, they are also employing folks seeking to maximize those dollars. Keri suggests that a “brain drain” of Friedman and others from Tampa’s front office to other teams hurt the Rays more than anything.

“There are no big secrets anymore,” Keri said. “You can develop your own data. You can come up with your own systems to try to prevent injuries and stuff like that. But it’s not like you’ve got this Luddite team over here and then this other team that is genius when it comes to numbers. … They all crunch numbers.”

So, where does that leave the Mariners?

If you have not developed a core of young, cost-containable players, you generally must spend more to acquire missing talent elsewhere. Or, you write the next five years off and see whether subsequent drafting and development produces a better young core.

The Mariners spent huge on free agents Robinson Cano and Nelson Cruz and on extensions for Felix Hernandez and Kyle Seager. And that is just to become good enough to knock on the second wild-card door.

But it has not been enough.

Now, rather than spending more big money for additional star players to put them over the top, the Mariners’ fiscal strategy appears this: accepting that their big-money quartet is playoff caliber and adding just enough value in less-sexy remaining roster spots.

In other words, finding that “Extra 2 Percent” in spots 6, 7, or maybe 8 through 25.

Keri rightfully says the Mariners could have done far worse than throwing money at Cano, Cruz, Hernandez and Seager. He sees Dipoto continuing to work “around the margins” to improve overall depth that typically does not cost as much in free agency, or trades.

Keri likes the Taijuan Walker trade that netted shortstop Jean Segura and others.

But he does not think Dipoto will part with more top young talent. After all, the Mariners still must rebuild on the fly to a degree as they try to contend, and there was not much on the farm already.

That explains Friday’s trade sending Seth Smith to the Baltimore Orioles for Yovani Gallardo. Smith is 34 and his future in Seattle was limited.

Keri noted that the Walker trade left a depth need in the back of the pitching rotation after Hernandez, Hisashi Iwakuma and James Paxton. So, Gallardo fills that need, which was deemed of greater value than Smith’s platoon outfield contribution.

Just like signing Carlos Ruiz gave depth at a catching spot that has also been a weakness.

It is hardly a flawless business approach. Rather, one of necessity unless the Mariners choose to leap into the top-10 payroll realm.

Things could unravel if either Hernandez, Iwakuma or Paxton gets hurt again or declines. Or, if Cano or Cruz shows typical mid-30s regression.

Even if the core holds, it is possible the team’s gains further down the roster are only a fractional improvement — or a break-even proposition. That is the risk when, as Keri noted, it is now tougher to one-up opponents on the value scale.

The good news, as shown this past fall, is that the Mariners were nearly a playoff team.

For now, though, instead of attacking that remaining barrier with a financial sledgehammer, the Mariners are using a chisel. And for that to work, they will need any modest gains to be a lot closer to an “Extra 2 Percent” than 0.2.

The Mariners spent huge on free agents Cano and Cruz and on extensions for Hernandez and Seager — just to become good enough to knock on the wild-card door. It has not been enough.