You get dust value automatically for the cards that are being moved from standard to wild, without having to dust said cards for any of that dust. You will get dust for duplicates if you can put duplicates of that card in a deck as well, but any more than that does not count towards additional dust. Blizzard is basically giving free dust to accommodate the change and also encourage more Wild play, instead of requiring players to dust their wild cards to get that dust.
So basically,
if you had Ragnaros, Sylvannas, and Ice Lance you would get 3240 dust when the move happens without having to dust any of those three cards (1600 + 1600 + 40).
If you have 2x Ragnaros, 2x Sylvannas, & 2x Ice Lance you would get 3280 dust with the move (1600 + 1600 + 80. The extra legendaries wouldn't count towards more dust since they go beyond the number of those cards you can include in a single deck, but the second Ice Lance can be included in the same deck so you get additional dust for that copy.
If you had 2x Ragnaros (one normal, one golden), 2x Sylvannas, & 2x Ice Lance (one normal, one golden) you would get 5240 dust (3200 + 1600 + 400 + 40). For the golden Rag you would get value from the higher card (the golden one over the normal version), but you would not get additional dust from the normal rag since you can only include either the normal or the golden version of the same legendary in a deck. You'd get the dust from a single copy of Sylvannas. For Ice Lance you would get dust value from both the normal & golden version since you can include two copies of that card in a single deck (If you had two golden Ice Lances & one or more normal copies of the spell when the move happened then the dust value would count the two golden copies over the normal one in that case, while ignoring the normal copies).
Basically you want to craft golden versions of the legendaries if you have the dust, or at least the normal copies if you lack the dust. Also craft two golden copies of the common and rare cards so that when the move happens you get refunded everything you spent on those crafted copies (as much as what can be fit in a single deck) without having to disenchant anything for those cards/dust.
Hope that helps.