I thought it was the cost of any is over 150% the original in just that resource...
Meaning, if you have a ship that costs 10.000/2.000/3.000, then retrofit it to something costing 14.000/3.500/4.000, it can't be done because the organic cost must be three thousand or less.
And I'm fairly sure there isn't a -50% in there. So with the above example, you could retrofit that to something costing 1.000/1.000/1.000 with no problem. The only reason the rule was put in was to prevent building a cheap ship then retrofitting to a very expensive ship (less time to build). Total cost wouldn't cover this, since someone could attempt to even out the costs of everything, then retrofit to something heavy in only one resource (10k/10k/10k to 43k/1k/1k for example). -50% wouldn't apply since there is no "cheat" for doing something like that.
That's what I think anyway, I haven't been able to play for approximately 2 months because my personal computer isn't exactly working
Oh, and geo is right too. Total cost would mean all three added together