Let's see. This is going to be the optimal number for bid winning before the win% decreases. For example, bidding 1 and winning the bid will give a huge winning percentage, but that is not going to be feasible. Obviously, the question becomes how high the number can be before the win% decreases and everything about equalizes.
It must be less than 16d, but more than 8d.
I think it is going to be about 11d-12d before there is a diminishing return.
$8/6vp = 1.33 per vp, and $1.33 per vp * 8vp is roughly $11. but the flexibility of debt allows you to go a little further than the optimal cost, as well as the junk card not located in your deck for the VP, leading me to the answer of 13-14d as the equalized optimal answer.
I will say it is likely 13d, but 14 is not much worse.