Much like “Fight Club,” Hollywood’s first rule of tentpole budgets is “Don’t talk about tentpole budgets.”

So how come Sony has willingly copped to a $258 million pricetag for “Spider-Man 3” — the largest budget ever acknowledged by a major studio?

The key word is “acknowledged.”

Ever since 1980’s “Heaven’s Gate” made big budgets synonymous with irresponsible management, it’s been standard practice for studio execs to hold the line on budget perceptions — not the budgets themselves, of course, just the perceptions.

A decade ago, $100 million was considered the benchmark for a pricey pic. Anything above that figure was disavowed by studio honchos. So even though the working budget for 1995’s “Waterworld” was $160 million, the “official” budget acknowledged by the studio was $95 million.

Three years later, there was much handwringing when “Titanic” doubled its budget to hit the $200 million mark. But it was clear that the old $100 million ceiling was shattered, as the mark quickly crept to $150 million and then $200 million.

This summer, despite studio chieftains’ vows over the past year to cut costs, the threshold could well be $300 million.

One studio veteran of the annual budget-guessing game declares, “People say, ‘Can it be told movies really cost $200 million?’ We should be asking, ‘Can it really be told they cost $300 million?’ because that that’s the truth.”

For Sony’s webslinger franchise, a huge sum like the admitted $258 million may be pricey, but it’s considered more investment than risk, since the first two films took home $1.6 billion in box office alone, before DVD, ancillaries and merchandising were added in.

But some skeptical studio rivals say “Spider-Man 3” has actually topped the $300 million budget mark.

The whole guessing game provides a timely reminder of the other rules of talking about tentpole budgets:

1) Take all figures with skepticism. No matter what a studio avows, rivals say the real budget is at least one-third higher than the reported one.

2) Time heals everything. Last year, Bryan Singer said on TV that “Superman Returns” cost $250 million. The studio and the director immediately said, no, it’s under $200 million. Now that all the returns are in and it’s made a profit, they’re more open to acknowledging the high figure.

3) Keep it all in perspective. Back in the 1950s, nobody talked publicly about budgets because most outsiders didn’t care. But when TV threatened to erode filmgoing, studios actually exaggerated the costs of movies: “Filmed with a cast of thousands! At a cost of millions!”

4) Only the accountants know for sure. With Hollywood’s famously pliable bookkeeping, studio accountants can easily make a budget come out higher — or lower — than the real tab, depending on the studio’s needs.

This summer, “Spider-Man 3” isn’t the only tentpole pushing the budget envelope. The latest installments in the “Pirates of the Caribbean,” “Harry Potter,” “Rush Hour” and “Die Hard” franchises, as well as “Evan Almighty” and “Transformers,” are all drawing scrutiny.

Universal is already weary of defending its $175 million budget for “Evan Almighty,” which some consider too high for a comedy. The studio counters that the “Bruce Almighty” sequel is designed as a four-quadrant film, and therefore poised for bigger B.O. returns than typical comedies.

New Line’s assertion that the “Rush Hour 3” budget is $125 million seems suspiciously last decade — especially after factoring in the cost of reuniting stars Jackie Chan and Chris Tucker, with director Brett Ratner six years after the series’ previous installment.

The meter is still running for “Pirates of the Caribbean 3,” which has been bedeviled by costly overruns and last-minute f/x work leading up its May 25 bow. Pic’s actual tally is hard to compute given the overlap between the second and third installments in the franchise, but before the overruns, the budget for both sequels was already closing in on $500 million.

Studios execs are quick to assert that shooting back-to-back installments in a franchise inherently brings savings. But that only blurs the line further.

For example, do the costs of the gigantic sets of “Harry Potter” go to the first film, or are they amortized over the life of all seven pics?

At least those pics have something to amortize. The CGI robots of “Transformers” had to be conjured from scratch — a fact helmer Michael Bay seems to embrace as a badge of honor on his blog, where he notes that the shots entail “some of the most difficult renders in ILM history. Sometimes up to 38 hours per frame.”

Thirty-eight hours per frame?

No wonder DreamWorks is sensitive about how much bang it’s getting for the $150 million budget Bay has acknowledged.