Amulek now exposes Zeezrom's ploy. Amulek tells Zeezrom (and not coincidentally, the crowd) that Zeezrom had no intention of paying off the bribe had Amulek accepted it. To translate the situation into one that makes more cultural sense to us, we might have a modern Zeezrom who is well-to-do, but certainly not a multi-millionaire, and an Amulek, who is also well-to-do. When the modern Zeezrom offers a million dollars, those who know him would understand that he really didn't have the wherewithal to make good on the offer. What he would be doing is tricking Amulek into saying that he accepted, whereupon he would be arrested on other grounds, such as false preaching, or whatever other law the modern Zeezrom might find.
This is the situation in Ammonihah. Six onties was clearly considered a large sum, and (as noted above) was more probably a psychologically large number as well as a number large in value. Amulek's acceptance would have signaled his lack of dedication to the cause he had espoused, and by having exposed Amulek as a fraud, Zeezrom could have imprisoned and discredited him without having to deliver on the promise. It is for this reason that Zeezrom began the "questioning" with this very important beginning. It would have ended things very quickly, and cost him nothing.