I don't know if this is up to date, but...
http://trainworld.us/NF2/English/shipyard.html

(5) “New” and “Sell” button

- You can buy a new ship by clicking the “New” button. If you want sell your old ship then click “Sell”. Once you have sold a ship you will receive 98% of the total number of credits originally used to buy that ship.

That is very outdated. you get 100% refund for the hull (I just bought and sold a yorktown without adding anything and credits did not change)

You get ripped off for Guns, FCS, Engine but not armor, ammo, aircraft.

The Buy/Sell prices are listed in the components section of the "game info tab" up top, e.g. 36,000 / 9,000 for the CV engine. The higher the tier component the more of a ripoff you get.

Just add up the price of the hull and the "sell" price for engine and fcs and since those are the biggest chunks of credit.

I always replace the engine with a ss engine before selling :P