This is indeed a potential issue with any proof-of-stake blockchain.
That being said, the price to pay is most likely larger than $8 billion as when the billionaire starts buying, the price may increase quite a lot.
Furthermore, many Algo-holdees hodl their Algos and will not necessarily want to sell at all.
Note also that a similar attacks apply to proof-of-work blockchains: instead of buying the tokens, you pay the miners and/or buy enough mining equipment to get 51% of the mining power and fork the chain.