In port, in the EQUIPMENT interface tab, in the Modules section, the displayed research cost of unresearched Gun Fire control System (GFCS) modules is frequently incorrect due to being "sticky" when switching between ships on the bottom carousel while remaining in the EQUIPMENT interface.
The research cost displayed on-mouseover, the cost displayed in the modal dialog after clicking the RESEARCH button, and the cost actually charged all appear to always be correct.
However, while in the EQUIPMENT interface tab, if you click around between ships (especially between ships set to "Primary") using the bottom carousel, the displayed research cost for the GFCS module is frequently not updated (i.e. it shows the research cost for the GFCS module of a previously viewed ship).
Screenshot of DD VIII Akizuki with GFCS module shown as 30 000 instead of as 8 000; I previously viewed DD IX Ostergotland whose GFCS module costs 30 000.