It's the exact same ratio of BE saved per RP spent as a 6300 tier champ and we know that Riot's customers spend money on that. It's a better ratio of saved BE than you get from buying 4800 and below champs with RP and some customers even engage in that practice.
Even if players don't engage in buying rune pages with RP over BE despite it being equal or better value than the champs they are willing to buy with RP, that still indirectly encourages customers to buy champs with RP by diminishing their stocks of BE so Riot gets their money anyways.