Most of the value of vouchers comes from the promo cards and a bit from the SPS airdrops that comes from the 300cp per pack you get. Vouchers will not keep that value after presale and they will give double the vouchers.
Theorically, vouchers should be much less valuable but what I understood with the presale here is that:
1- FOMO is really hard
2- Team is pushing as much as they can to keep value of vouchers saying that they will be required to get all the best things in the game in the future even if they still don't know what it is.
3- People spend like crazy and they don't even know what they are doing and aren't any good at maths.

With all those things in mind, you can be sure that the price of vouchers, even if it drops, will never drop as much as it should. Therefore, I don't think that the value of vouchers will drop any further than 4$ each and I wouldn't be surprised to see 6 or even 8$ at some points which is ridiculous considering that you will be able to buy packs at 4$ just after that.

Agroed said that he thought that there would be enough packs for those that want to buy and that he would be surprised that they sell quick. IMHO, I wouldn't be surprised to see them disappeared within a few hours if the servers can take the load.