Handling maintenance and overhauls for GURPS Spaceships

[Calvin]

I'm going to be giving my PCs a ship in an upcoming session, and I'm trying to figure out the rules for maintenance and overhauls.

My general idea was to give them a Cheap ship so that fuel and maintenance costs would offset any income from moving cargo so that PC wealth doesn't get out of hand. I've also got general upgrades outlined for them (removing some unneeded modules, adding habitat modules to empty spaces, replacing cargo bays with something else, etc.) but I'd also like to give them the option of overhauling the existing modules.

Basically the idea would be to let them repair a module to the point where it stops being Cheap, and so stops counting for maintenance purposes. But what would this cost? The upper bound for what it costs is simple, it's 80% of the module's normal cost. If it was 80% or more it would be cheaper to simply sell the module (At 40% of the cheap module's price) and buy a brand new one. The lower bound has got to be 50% since if it were cheaper it would be possible to buy a cheap module and then overhaul it to standard quality for less than the price of a new module. Are there even rules for this?

It is possibly stretching the bounds of reality, I know that it might very well be too expensive to get something back in order if it's in really terrible repair. But I would like it to be an option for the PCs, if only to create a money sink to handle wealth inflation.

As a related question, does it make sense for the engineer PC to be able to reduce maintenance costs by their monthly wage per month worked on the ship?