irishguy28

While not impossible, it would be unusual in many cases if the cost of each sector in a return ticket were the same.

Ticket are not sold on the basis that a return ticket is always made up of two equally priced components [though, in some cases, it may be possible to select two equally priced components, but this is the exception rather than the rule]. As such, your assumption that your refund should be half of the amount paid is incorrect.

This is totally logical. Think about this example: let's say your wife chose an early Monday morning outbound flight, that is already practically sold out and which is therefore an expensive flight, but chose a Saturday night return flight, for which almost nobody has booked a seat, and which is therefore a very cheap flight. Would you be happy that she is charged the much higher, almost-sold out fare for both sectors, solely to preserve this "symmetry" of ticket pricing? No - you would not be happy. Your wife would be paying far over the odds for her deserted return Saturday night flight.

Additionally, the taxes associated with each sector are often (vastly) different. Each country and each airport can potentially have its own fees and charges - and some countries [such as the UK] charge hefty duties on passengers departing their territory by air. Any such taxes/charges are uniquely associated with each sector of the trip. You cannot therefore conclude that the total taxes are to be split evenly between sectors, either.

When you buy flights on the Air France website, you are shown the individual cost of each sector as you step through the process. For example, looking at AF CDG-AMS tickets for next month, here is the selection for the outbound (first image), and then the selections for the return (second image)

On the outbound, you must select a fare of either 59 or 82 euro, and on the return you must select a fare of either 51/217/70 Euro. This means your total ticket price will be either 110, 129, 133, 152, 276 or 299, depending on the specific combination of fares you select. Halving these totals gives, respectively, 55, 64.50, 66.50, 76, 138, and 149.50 - none of which corresponds with the price of any of the return components as shown below.

In fact, it is probably incorrect to even assume you should get the full value of whatever price your wife was presented with for her return sector, and selected, at the time of buying the ticket. Usually there is a service/handling fee, and they may even reprice the ticket to the corresponding one-way fare, and only refund whatever balance is left over.

But let me assure you - it is not at all unusual that the refund is less than half of the total paid, and I can assure you that it is highly unlikely that any error was made in coming to this amount. I would advise just dropping the issue and accepting the voucher as being correct.