There's a rather illogical game mechanic where destroyed ships will drop copious amounts of metal filling up your cargo holds but even if your Onslaught gets all of it's armour blasted down to black and has 10% hull remaining you never need a single piece of metal to repair it.
Instead Supplies, which in my mind are a mix of food, spare parts, raw feedstock for 3d printers, replacement circuit boards, pipes and tubing, space suits, programmable drones etc. are the only thing that gets used to repair a damaged ship. I think it would make the game a lot more fun if we could change this a bit so that readiness continues to consume supplies as normal but repairs consume a mixture of supplies and metals! This will also mean that metals actually become useful rather than cargo-filling vendor trash.
The following is based on the assumption that the existing repair cost = 100% supplies and any figures given are relative to the existing repair cost. For balance since we are adding metals costs we should reduce the supplies costs slightly to match.
Low Tech Ships - 65% Supplies, 65% Metals
Midline Ships - 75% Supplies, 50% Metals
High Tech Ships - 75% Supplies, 35% Metals, 5% Transplutonic Metals
Alternative Option:
Low Tech Ships - 50% Supplies, 65% Metals, 10% Heavy Machinery
Midline Ships - 60% Supplies, 50% Metals, 10% Heavy Machinery
High Tech Ships - 65% Supplies, 35% Metals, 5% Transplutonic Metals, 5% Heavy Machinery
I think in terms of costs it should be reasonably comparable to current repairs costs while making repair costs seem more realistic and also make it so that some of the less often used inventory items become useful again.