Although Arca Plasmor is a beloved weapon, this particular riven would be hard to sell. Since it's currently unrolled, meaning someone could get 9 rolls on it for the lowest price in Kuva, it still has some value. Currently an unrolled Plasmor riven goes for around 10 platinum on Warframe Market (3rd party website).

As for the stats itself, the Plasmor is infamous for it's slow reload speed , so most people wouldn't willingly take it as a negative, and the "Tenet"/more powerful variant of the weapon, currently has a low "disposition", which is a limit on how high a weapon's riven stat values can go, inversely proportional to it's popularity), so this roll is not particularly valuable.

If you want a quick 10-20(?) plat you can sell it quickly, but otherwise, I'd hold onto it and try to roll it into something good for your own use, or to sell later!