I would expect monthly lease rentals for a relatively old ATR42 to start somewhere around USD40,000, which - very rough 1%/month rule of thumb - would put the equivalent purchase price somewhere around USD4m, or £2.2m.
I've looked up the original reference and suspect it might mean "£200k to be used as initial lease deposit on an ATR42" (a few months' rentals) rather than "£200k to buy a whole ATR42".
(note: considerable variation in ATR42 rental rates: monthly lease for one of the newest ATR42-500s could be about 3 times that of the oldest ATR42-300s. So take the above numbers just as indicative - I don't know what type/age of ATR42 Aurigny is - allegedly - going after).
C.