Prestige Blood Moon Aatrox costs a hundred prestige points. You can, as stated above, only get those in bundles.
If you didn't buy the gemstone pack yet (which includes 20 Prestige points for the best possible price in the store right now, 1750 RP, but is limited to one purchase per account), the total (1x Gemstone Pack, 2x 9 Blood Moon Capsule Pack) comes to 15250 RP. As a comparison, Elementalist Lux costs 3250 RP.
Now, if you pay this using a method that doesn't yield any bonus RP, you will have to put down the nice slim sum of ~122€ in Europe, or if converted to dollars: 140.28$.
Seems worth it for what is essentially a golden Chroma and not even an ultimate skin, amirite?