Nice thought, Loupdinour, but you're missing the point. An NLC costs 500 to build. Rush buying it costs 5580bc.
It seems to be BuyNowCost = 6 * BuildCostFactor where BuildCostFactor = BuildCost ^ 1.1, rounded down. i.e. 500 = 930 x 6 = 5580.
Incidentally, the Overlord design in DA costs 400 to build, and 4368 to rush buy...even when I'm neutral. So I'm going to call that a bug. Neutral in theory gets a reduction on rush buying ships, but apparently not planetary improvements. I'm seeing neither in DA 1.80g. Just checked in TA, and it implements it as a PurchaseNowReduction i.e. InterestRateBonus, and works correctly-for both ships and planetary improvements. I am unaware if the latter is intentional.
Point being, in the examples Boogly provided, you'd need a 97% buy now reduction-for the NLC. I can only assume that constructor isn't the default one, though, as a 97% buy now reduction places it at 47bc (from 1578 initial BuyNowCost at 159 initial BuildCost) rather than the mentioned 40bc.
I'm guessing that constructor costs between 137bc and 139bc. This would also place your medium ship at 330-332bc to build, rather than rush buying. Feel free to correct me if I'm wrong, Boogly.
One other thought-I've sometimes run into planetary events that provide an InterestRate bonus, and, if I remember correctly, also a couple of anomalies. This appears to be the BuyNowReduction we have been speaking of, based on the values the TA techs in question provide.
Still, 97% is a heck of a lot.
Could you check your Stats & Graphs screen for Abilities, Boogly? Would be interesting to see what it reports, as far as interest rate goes.