Sounds like you've got a good approach on the cost/benefit calculation and just lack one of the variables.
My own findings are confirming that information on specific insurance company defensive driving discounts aren't generally disclosed, and discounts aren't consistent across companies. Some insurance company pages I found do confirm many offer a discount, but not exactly what percentage that discount would be. Further browsing revealed:
an anecdote from one driver: "they offer a 10% discount if I take a defensive driving class"
a mention from one training company¹ that "Some insurance companies give a reduction in rates to course participants" – meaning, other insurance companies might not!
Wikipedia: "Courses may also qualify the student for a discount on car insurance." [my emphasis]
An answer elsewhere stating "varies [...] but discounts of 5-15% are common."
So if we told you "between zilch and 15%", does that satisfy or would you still be wondering? ;-)
I think you need to seek out information specific to your case. IIWY, I would call my car insurance company and ask: "If I qualified for your defensive driving discount, how much would I be paying for my policy, all else being equal? What specific or kinds of courses would qualify me? Online, or in-car only?"
It's in their interest to inform you on this, since better trained policy-holders are less likely to claim.
¹ Note: I don't endorse any particular company; this was just an example from my search results.