I don't believe it.
Most of the player profile sites I have looked at show his previous transfer fees as 'undisclosed' and say his Feyenoord contract was up in summer 2016 (after 4 years). Transfermarkt quotes a fee from Feyenoord to Cardiff in May 2016 of £1.96m - but that looks like a totally made up figure and so specific that it seems someone was trying to make it look real.
http://www.transfermarkt.co.uk/lex-i.../spieler/56371
Another thread on **** is claiming Immers had a year left on his Feyenoord contract (I can't see any references to it being extended, but maybe) and that Cardiff paid £500k.
All speculation but I doubt we paid much. It was a quick signing whilst he was still under contract with Feyenoord (either a few weeks or a year and a few weeks) because the embargo had just been lifted and because other Championship clubs and Rangers were interested after his loan performances.
It is all speculation.