No offense intended, but using a financial calculation to justify (or not) an investment in your future freedom somehow strikes me as silly. I understand your logic, and of course the entire calculation is dependent on your life span expectation. That's not the point.
Over a period of years (not even sure how many - maybe 25), I've gradually worked my way up from annual to Life to Patron to Endowment to Benefactor. I assure you that, along the way, I've made donations over and above my payment required to reach the next level.
I've lived through numerous tales of doom and gloom. I lived in California around 1980 when a ballot initiative caused a run on guns bigger than what we're seeing now (I forget the proposition number, but it was defeated - but that was long before California became California).
The NRA has always been there. No, they aren't as radical as many people would prefer, nor or they as uncompromising as other RKBA organizations. But that's why they succeed. They know when to back off and when to attack. No, they don't always get it right, but if they were perfect, we wouldn't need them. Spend the dough for a Life membership, then work your way up. Is it a good financial investment? Ask your great grandchildren the next time you dream about them.