How much does it cost to get the Prestige Riven skin?

It's an event skin, so you'll have to pay $15 then play a few hundred games to grind out 2000 tokens which will be used to purchase her.

Actually we don’t know the cost yet for the skin and main event skins are usually the ones you have to grind out. Like the Arcade event,project event and KD/A event. Im assuming this skin is probably going to be a prestige point skin that you have to have 100 tokens for because the immortals events are even less hyped that the rest of the events so far this year