(snip)

Well, I googled "converting freighter to passenger" and what popped up on top was a number of YouTube videos about converting airliners between these configurations. Based on viewing one of those, my principal thought is that cost would depend very much on whether the vessel was designed to be (more or less) easily converted - I assume that (of CT designs) the Sulieman, Lab Ship, Liner and probably the Yatch are so designed - and how often this is actually done. If the conversion you want to do is easy and common, then there are going to be aftermarket packages available at a discounted rate (at least at Class A ports). But if you're doing something unusual with an oddball ship, you'll be on your own.

As a general rule, in games I've run, I generally charged half the cost of a what the implaced items would cost if building a new ship, for a dockyard conversion. This covered the labor involved in removing the unwanted items and installing their replacements, including any incidental materials and structural changes required. I used the new construction cost as my base, because I figured that at least 25% of that cost was for dockyard Iabor. I then doubled this to allow for taking out the old stuff. 

For penny-pinching PCs, half cost *assumes* the dockyard retains the removed items; cost increases 20% (e.g an additional 10% of the new parts cost) if the PCs want to keep the removed items. For *really* tight-fisted PCs - who have the appropriate skills - the labor can be done by the crew (using the rules for annual maintenance, figuring the time required as if the dockyard asking price were the regular cost). In this case, the only cash outlay would be rental of a dockyard slip plus appropriate insurance.

(Of course, if you need to BUY the new components, those will cost 75% of their cost as part of a newly constructed ship.)