Starships in Starfinder feel like a second thought mechanic. Even with the addition of the later Starship Operations rulebook our group felt like their primary transportation and base of operation was less emphasized than it should be. Our group developed the following rules to make starships more front and center in this star faring, sci-fi setting.
Estimated Ship/Subsystem Value: 1000 credits per Build Point *
This is a baseline from which negotiations can begin. This method of valuation likely skews poorly when you consider the resources involved in building Huge, Gargantuan and Colossal ships of any tier. Construction, ownership and operation of those ship types is typically limited to planetary, military or corporate authorities so cost is usually irrelevant to players at that scale.
Because the Tier of a starship roughly determines the space available and technological complexity for installing subsystems it is reasonable to associate the total value of a ship based on the pool of build points designated by the ship's tier.
Brand new ships, right off the assembly line, generally start at around 200% of base market value, further adjusted for factory fitting and trim.
Negotiations to sell a clean and clear titled ship begins at 30% of it’s estimated value.
Stolen ships are typically valuated at 10% of their value due to the legal entanglements.
Salvaged ship frames are usually valued based on what remains functional and how much it will cost to recycle, refit or retrieve the frame.
The "Build Point to Credit" formula remains a reasonable rule of thumb to determine the cost and value of upgrading or scrapping a subsystem. The percentages noted above for new, used, stolen and salvaged subsystems should make short work of transactions.
Yes, you could sell existing subsystems to offset the cost of new subsystems if you really want to grind those numbers.
Player characters can influence the variables of buying hardware of all kinds. This usually involves either a Diplomacy or Bluff skill check. It could also require some rousing role play as well.
Per the Core Rule Book, selling loot will net the players 10% of the market value with no negotiation. This is not explicitly applied to ships as there is no core valuation system included. We apply it as a house rule.
The DC for negotiation is 10 + Class Level + Ranks (Diplomacy/Bluff) of the NPC negotiating with the players. Negotiations are typically one-on-one so there is no assist role. Characters can get a bonus based on their Ranks in Engineering, Computers or Piloting (one, not all).
This can apply negatively to failed rolls just as easily.
This is a loose and fast set of negotiation rules. Tailor them to your game accordingly.
* Pricing based on build points is purely speculative. A ship is only worth what the market will bear.