Depends where you're flying from, but use http://www.opodo.com or google for cheap flights and see what you get.
If you're prepared to use one of the London airports, then it may well be cheaper as there's more competition for customers, but it shouldn't be too expensive from most places in the country
As an air traffic controller I know there are definitely services from Blackpool to the Isle of Man with Manx2 at http://www.manx2.com - they're quoting around �30 return. The also fly from Leeds, but it's around �100 a ticket then.
Flybe go from Liverpool to Ronaldsway for around �60 a time, take your pick.