set in stone

UG先生，皆様，明けましておめでとうございます　今年もどうぞよろしくお願いいたします

ジムに通う決心をした場合は何月に入会するのがいいのかという記事です。
Is the New Year the Best Time to Sign Up for a Gym Membership?

However, Keightley notes that gyms will no doubt be packed with other members -- perhaps a 30 to 40 percent increase in traffic -- who signed up as part of their New Year's resolution.

"Be patient and be prepared that it's not the little quiet gym you can go anywhere you want," he said, at least until March, when foot traffic will likely flatten.

Keep in mind too that gym prices are highly negotiable. Once a fitness club has its doors open, each new member adds little in terms of costs, so prices are often not set in stone.

http://abcnews.go.com/Business/best-time-sign-gym-membership/story?id=21373583

記事の中に出てくる”set in stone”を『Wisdom英和辞典（第三版）』（三省堂）で調べると「全く変わらない，不変の」という意味が出てきました。ここのsetはinscribeと同じ意味で、石に刻むともう変えることはできないというところから発展してきたものです。

ジムの入会価格は日々変動しているので交渉次第で金額が変わるということが書かれています。（Ume）