They have never put character slots on sale before. Recently they have started doing small discounts on seemingly random crown store items so maybe there will be a discount in the future.

In my opinion extra character slots are the best value item in the crown store, nothing else has so much potential value. Cheaper than most mounts, and all motifs, a new character not only lets you experience the game freshly again (including letting you get more one-off items), you can quickly make money off of a new character slot. Extra character slots are the only item that I think could be considered pay to win, because the more characters you have the more gold and items you can make by doing easy dailies, through hirelings, etc. Anyway, I have 15 max level characters and will buy more as soon as they are available.

Thank you all