Two stamps, not one
Ghana has not joined the Hague Apostille Convention. Where a country sits outside that agreement, the older chain of stamps carries on as before, so a British sheet is marked here and then stamped again by consular staff before anyone abroad will read it. The British mark is genuine and useful. It is simply the first half of the trip rather than the whole of it.
Who pays which rate
The British side publishes two rates, and the difference between them is not about the paper at all. It is about the sender. The standard postal rate is open to anybody. The lower next-day rate is open to registered businesses, so a company moving staff to Accra and a family moving there are not on the same footing even with identical sheets in the envelope.
That gap tells most where volume is involved. One certificate barely notices it, while a firm posting a folder of company records, staff degrees and police checks meets it on every sheet, because the charge attaches to each document rather than to the packet. Working out how thick the pile really needs to be is the first honest piece of budgeting here.
The half the British figure does not cover
An apostille from United Kingdom to Ghana buys exactly one stamp. What consular staff charge, and how long they take over it, is settled on the Ghanaian side and forms no part of the British number. So the sum you can total at your own kitchen table is real but partial, and a plan resting on it alone will come up short on money and on days alike.
Taking the job in the right order
Three moves, in this order. Settle what Accra actually asked for, so nothing is marked twice or marked for nothing. Settle who is sending, since that fixes which rate is open to you. Then get the sheets into the post. A file put together that way rarely needs a second envelope, and it is the second envelope that really costs.