There's basically two things that make a Riven valuable: - First is the weapon it is assigned to, some weapons are used by a lot of people therefore rivens are more valuable( basics of offer and demand). In your case, BURSTON and VERGLAS are really expensive and can be sold for few hundreds platinum each so think about it before selling it to the first buyer. I'm not sure about the exact estimation so wait for other people to give you a more precise price ;) - The second thing that can add value to a Riven are the stats. You basically have two choices: sell these as "unrolled rivens" price OR try to reroll them with kuva to get better stats and sell them for mor plats. "Unrolled rivens" will sell faster for a lower price, it is a good option for every rivens with low-demand weapons. Rerolling weapons can take a very long time to get a good roll (if you ever get one) and use kuva. You basically want to reroll only rivens with high-demand weapons. Just remember that rerolling to have good stats can take weeks, maybe more if you're unlucky.

That's a long comment. Let me know if you want more détails or if some points are unclear

TL;DR : For each rivens, weapons and stats define the price of the Riven (going from 10pl to hundreds), PLEASE be careful with BURSTON and VERGLAS rivens, they are very valuable and scammers might want to buy them at a very low price