I notice that flights to Ghana from Europe seem to be much more expensive for stays in Ghana of less than seven days.
I'm familiar with European flights that are cheaper when they span a weekend but have not previously seen this as a factor.
It's easy to see the effect here on Skyscanner
Is this typical?
It's not unusual. Very short stays tend to be the domain of the business traveler, who can afford and can often justify paying the higher fares. Leisure travelers and family travelers, on the other hand, usually stay longer and are more price-sensitive, so won't pay as much.
It wasn't that many years ago that domestic North American fares tended to be cheaper with a Saturday night stay (business travelers tended to go home for the weekend). Even today, Wednesday and Saturday departures are often cheaper precisely because they are less popular among business travelers.
Adding to Jim's answer - if you do a lot of short-term traveling to a given country and want to circumvent the airline's fare pricing, you can use a technique called "nested ticketing" to fool the airline into thinking you're staying at your destination for longer than a week.
You can read more about "nested ticketing" on various blogs online, such as this one.
