How does that make generating vanity addresses cheaper? You still need the same amount of processing power to generate one vanity address.
Yes, the more computing power, the better.
However, you can generate many vanity addresses at once. Accordingly, the more simultaneously generated vanity addresses, the greater the chance to find one of them and the less its cost.
It all depends on the chance: the more the chance to find a vanity address, the cheaper it is.