Not enough info.
$20 for 50 rounds of HP might be a screaming deal depending on the ammo. Good quality HP ammo is $1/round or more. If he is selling HP at $20/50 rounds it is either really cheap HP ammo or a fantastic deal. If it's not quality ammo I'd just put my money in the FMJ. If it's the good stuff buy $100 worth and buy FMJ later.
Around $12/50 rounds is about the going rate locally for new FMJ 9mm ammo so that isn't really a deal. I can do as well or better. One of the local ranges is selling 9mm FMJ at $10/50 rounds, but only if you buy an hour of range time at $10/hour.