You can pickup a 1TB Crucial MX500 for under $100 USD and a 2TB for just over $200. There's cheaper options if you opt for DRAM-less SSDs like the BX500 but with tradeoffs (warranty, overall performance, lifespan etc). They are still a damn sight faster than the stock HDD though.
I only play some platform exclusives on my PS4 pro so a 500GB SSD is enough for me. Your mileage may vary since the main bottlenecks are ultimately the interface it uses and if the game can leverage the improvement.