Translate only the documents relevant to your application, not the whole file; ask whether a certified extract is acceptable for your purpose; and send clean, full-resolution scans so nothing has to be re-quoted. Never, though, cut pages from within a document that genuinely needs translating.
There are legitimate ways to bring the cost of a long legal file down, and one trap to avoid.
The biggest lever is selection between documents. As above, you rarely need the entire bundle — so decide which documents your application actually relies on and translate those. Blank pages, duplicate copies and purely procedural notices can usually be left out of the count entirely.
The second lever is the certified extract. For some purposes a court or caseworker will accept a certified translation of the relevant part of a long document rather than the whole thing — the operative section of a judgment, say, rather than two hundred pages of annexes. Whether this is acceptable depends entirely on who is receiving it, so confirm it with the receiving body before you order, not after.
Here's the trap: selecting between documents is fine, but cherry-picking within a document that genuinely needs translating is not. If a document is relevant, it has to be translated in full — every page, every clause, every stamp. Partial translation of a document that should have been rendered completely is one of the most common reasons UKVI rejects translations, and a rejection costs far more in delay than you ever saved.
Finally, help your translator quote accurately the first time: send clear, full-resolution scans of the original. Blurry photos make stamps and endorsements ambiguous, which leads to re-quotes and back-and-forth — exactly the friction you're trying to avoid on a big job.