Over-simplifying hugely here - for roughly every dollar Dreamworld spent on Tower of Terror Kua made two dollars in the final sale. 2:1 ROI ain't bad.
I also don't think it was necessarily cheaper per-say - two parks jumped on un-proven technology which never worked 100% of the time to buy a one-trick pony that was extremely, extremely marketable in a pre-Google & less globalised world. Back then, punters took world-record claims at face value. Folks today are more knowledgeable, more travelled and more weary to gimmicks and all of those buzz-words (world class, precinct, activation, southern hemisphere's XYZ) which don't have the same shine they once did.
In my mind, in 2020, if you had 16 million in today's money (28-29 million) would you spend that on something roughly like Rivals which is from a proven manufacturer and will deliver results long term, or risk it all on something like Ring Racer at the Nurburgring?