Going through the spaceship building chapter of Starfinder I'm rather impressed with the versatility in there, but I seem to not be able to find what these spaceships cost. The header "Building a Spaceship" (p. 293) suggests that the first spaceship for the party is free, but if they want to obtain a different ship or need a larger/smaller one, the party would have to buy one. Except I seem to be unable to find what the price tags for these are. Do I keep missing something, or is this intentional on the designers' part?
It seems that Spaceships aren’t supposed to be monetary assets, but operate in a similar manner to how characters level up and learn new skills.
Page 305 says
As the PCs go on adventures and gain experience, they need an increasingly powerful starship to face tougher challenges. When the characters’ Average Party Level increases, so does the tier of their starship… The PCs receive a number of Build Points equal to the Build Points listed for their starship’s new tier – those listed for its previous tier, which they can use to upgrade their starship.
Instead of buying new parts and ships, the book suggests characters are finding salvage, making arrangements or calling in favours.
This is facilitated by Build Points that are granted according to the Average Party Level (see Table 9-1 on page 294, for players, the APL is equivalent to Tier, for enemy encounters, tier is the method of determining difficulty). Tables of parts and upgrades list “Cost (in BP)” that the players should sum up to design their ships.
From page 305, it appears that the majority of changes to ships will be via upgrades, keeping the total Build Points of components on the ship within the Build Points the party is allowed via their APL. Players add to the ship whenever they've got spare BP. The section on Upgrading Systems isn't too clear, but removing one part grants you all the BP that part is worth which you can spend on other parts.
The party can also "get a new starship" by rebuilding from scratch with a new base frame (say upgrading from a Light Freighter to an Explorer). While the rules don't say this explicitly, it appears that this functions like any other upgrade. If you get rid of your original starship, you have all your Build Points to spend on a new one.
Regardless of whether your upgrading or replacing your starship, Build Points aren't expended like credits. You "level up" your ship to match the Build Points your party has been allocated.