sadiemay, what dates are you looking at, what airlines have you tried, and where did you look for train fares?

There aren't many LCCs that operate between London and Paris, but you could try Easyjet - www.easyjet.com - who fly from LTN. LTN is "London" Luton Airport, but Luton is actually a town in its own right a fair way north of London. Still, if you don't mind the train journey there... but then again by the time you've got to Luton on the train you could be half-way to Paris on Eurostar.

Train fares start at GBP 29.50 one way (non-flexible), which is considerably less than the fare you were quoted - but if you're travelling around popular dates then the cheap tickets may well all have gone.