Hull plates are labeled as Titanium I, II, III, IV or something like that, i believe.
I dont think thats what OP meant. But probably "hull repair unit", which indeed, at Sapien, has an outstanding cost/space ratio of around $15k/1space. Followed by "Reactor Repair Kit" $12k/space. Great at start with no money. Titanium plates on the other hand have a ratio of 0.95. An exceptionally bad deal. Trellis is average with 7. All of them are beaten by SDM-Drogue@Earth though, with a ratio of 90 (OTS-Bugnor is 28)
In order of profitability
Planet, Part, "K money"/space
@Earth
SDM-Drogue 90(30)
OTS-Bugnor 28(9)
@Sapien
Hull Repair Unit 15
Reactor Repair Kit 12
Trellis 7
So best would be to ship SDM to whatever place with high inflation. At least thats as far as i discovered, and im not that far in campaign yet. Maybe there are even better deals.
Edited: Then again weapons use Weapon Bay which is usually 2500, while spare parts use Cargo Bay which is 7500, so this would bring the Drogue down to 30, and the Bugnor below the H.R.K. hmm.