Page counts vary wildly from file to file, so here are three typical ones at our standard rate of £15.59 per page.
Partner route, five years in. You have English translations of the marriage certificate and both birth certificates from the original application, done to the ILR standard. What's new: a child's birth certificate (1 page), an overseas rental agreement (3 pages) and six months of statements from the foreign account the rent lands in (6 pages). That's 10 pages: £155.90.
Skilled Worker, family of four. The original translations came from a provider that didn't state credentials, so they all need re-doing: marriage certificate (2 pages), both partners' birth certificates (2 pages) and two children's birth certificates (2 pages). That's 6 pages: £93.54. If those originals had met the ILR standard, the bill would have been zero.
Long residence, with two years abroad mid-route. An overseas employer's secondment letter (2 pages), foreign payslips (6 pages), a police certificate (2 pages) and a parent's death certificate explaining a long absence (2 pages). That's 12 pages: £187.08. If your current visa is close to expiry and you need the same file same-day, it's £29.99 per page: £359.88.
The quickest way to an exact figure is to upload everything at once and get a quote. Pages are counted per document, so you only pay for what's actually in the file.