I'm a lot more concerned with getting people to call than I am with getting people to fold. If we know that one of them has an ace and will call $12 but not $21, and that the other one has specifically a flush draw, AND that we're somehow unable to bet the river so whatever happens no one will put any money in on the river at all, the immediate
EV of a $12 bet vs. a $21 bet is this:
$12: 4 times out of five, we will win $48.
1 time out of five, we will lose $12. Expected value
: 0.8*48 + 0.2*(-12) = $38.4 - $2.4 = $36
$21: Every time, we will win $24. EV = $24.
Of course, there's more to this.
1. We can't be sure that A-3 folds to a $21 bet AND calls a $12 bet.
2. We can't be sure that a flushdraw folds to a $21 bet.
3. If a third club hits the river, we are likely to pay off some amount, even if it's hard to say how much.
4. We don't actually know that these are the hands that they have.
Still, the overall point still stands: I'm more concerned with getting value from the guy without a flush draw (presuming that at least one of them is NOT on a flushdraw) than worrying about the 20% risk of losing to a flush.