Implied Odds Question

[InTheDark]

$2-$5 NL - One limper, tightest player at the table raises to $20, Bozo#1 calls, Bozo#2 calls on the button then the Unknown Quantity, as SB, calls with 42s. Assuming this is my typical game and most everyone has stacks in the $100* range, this call is IMHO insane.

My question for those that know, assume you read people like Gus Hanson. What stack sizes would you need to justify this $18 call?

*Yes, I know this is short. It's the way my game is played.