Yeah, I've heard a bit about that one, but have lumped it into the commercial ones that you'd have to have a solid need to justify a license cost like running something important in production that requires support or you'd probably need an academic license.
The personal license cannot be used for any commercial purposes (basically just academic research or play). The single-user commercial license is over $2000. Not that any of that is unreasonable in industry, but you're making it out to be cheaper than it is.
$2250 is the first year. Then $750 a year. This includes support. To my understanding, development alone without distribution falls under the personal license.
As far as modern Prolog. I know commercial systems still exist, but I assume most work has switched to SWI-Prolog?