That offer is almost permanently available:
(From the Dutch-language version of the Flying Blue newsletters. The offer that currently runs and expires on 18 March ran last December with an expiry date of 31 December, having ran just previously with an expiry date of 13 November 2015)
Don't be fooled. As the previous poster said,
cash is a far more liquid and convertible currency than are FB miles.
Topping up your FB balance by a small amount might make sense, if there is a ticket you know that you want to buy and that is available to buy with miles.
Buying large amounts of miles at this "discount" (but it's not; when this is the "usual" price, then it's not a discount at all) is generally a bad idea - especially if you are not a frequent redeemer of FB miles and are therefore not perhaps aware of the difficulties you will face in redeeming them.
Think of it this way: the cash that you are apparently willing to trade for FB miles could be used to directly buy a ticket. And not just AF/KLM, or Skyteam, but ANY airline. Or it could be used to go to a restaurant. Or stay in a hotel. Or do your grocery shopping for the week. Or...etc etc etc
The FB miles you buy with that money, though, might not even be able to buy you the ticket you want on AF/KL.
You can always spend that cash. But when you convert it to FB miles, then you have drastically reduced your options. You can only spend it when and how AF/KL let you.
There will
always be tickets available for cash. But there are often no tickets available for miles - particularly if you are not flexible and you HAVE to travel on a particular route and on a particular day and on a particular service.
To use your miles to upgrade into business, you will have to buy a rather expensive economy class ticket in order to have a ticket that can be upgraded with miles, and then do a "double-upgrade" (remember - AF has a Premium Economy cabin, so you have to pay twice as much in miles in order to sit in Business rather than in Premium Economy).
Buying FB miles is such a terrible, terrible idea.