IIRC in the 90's the APEX return fare to London on Manx was £166, otherwise around £280. If you were a member of "Club Sovereign" you could book a spouse fare for your better half which ran out at around £430 return for 2.
In the Flybe days I used to think around £200 return was acceptable for a booking close to the departure date. BA was occasionally a bit more depending on demand. Loganair took £250 from me for flights this week (and charged me extra for my seat).
Since then we've had £20 odd of APD added plus ever increasing airport charges (LCY and IOM took almost £60 on a return before the pandemic). How airlines make money at that level is beyond me.