Happen to have a Manx Airlines 1993 timetable in the garage, and that's back in the days when airlines used to publish fares as well.
IOM-LHR was £103 Economy Single
£180 Day return
£168 Pex Return
£158 Apex Return
£134 Super Break Return
Using an on-line inflation calculator thingy, that gets you to £180 one-way, £315 day return, £293 Pex Return, £276 Apex Return and even the cheapest return at £234 in today's money. That's just straight inflation and no adjustment for APD, airport taxes etc.
Even the Farecracker to Luton was £122 return so £213 in today's money.
Looking at it another way, anything less than £200 return nowadays is a bargain compared to 1993 pricing.