5.7 has gone down in price with more guns coming out it in. Buying 20 rounds of defensive ammo in 5.7 in 2019 would have easily been much more than $30. Still less than $700 but probably more than just handing over all your cash
I want to say 5.7 went up 2019/2020 something crazy, there are old forum threads referencing pricing on Ammoseek from around that time, about $1.8 per round is the most expensive I could find.
But yeah, I didn't even check the date on the original post.
Defensive rounds like hollow points typically are more efficacious at stopping a threat instantly but they tend to cost more so people typically use cheap ammunition like FMJ for training or recreational use.