The half you can look up
The British charge is public and steady. The FCDO Legalisation Office asks £45 for each paper on the standard postal route, or £40 for each paper on the next-day service open to registered businesses. It does not rise because the destination is distant, and it does not fall because you are sending several at once. That is the easy line in the budget.
The half nobody publishes
Gambia has not joined the Hague Apostille Convention, so a consular stage sits on top of the British one and embassy legalisation is required before a file is usable. That stage sets its own charge, and no honest page can quote it in advance. Treat it as a line with a range rather than a figure, and the budget survives contact with reality.
Paying for the right copy
The cheapest saving on this route is also the least obvious. Edinburgh Registration Service at City Chambers, 253 High Street, Edinburgh EH1 1YP, prints certified extracts of Scottish births, marriages and deaths. Ordering a fresh extract costs little and keeps the record you cannot replace at home, which is a bargain measured against starting the whole chain again.
Carriage, the line people forget
Postage is where the quiet money goes. An apostille from United Kingdom to Gambia involves at least two domestic legs and one international one, and tracked carriage to West Africa is not the price of a stamp. Budget it as a real line rather than an afterthought, and move several papers in one go wherever the deadlines allow you to.
English, and what that saves you
One cost this route does not carry is translation. English is the official language of Gambia, so a Scottish extract can be read as issued and no second-language version has to be commissioned. Document legalisation for Gambia is therefore a shorter bill than for most of the region, provided the paper itself arrives clean, flat and whole.