The majority of airlines have been forced, due to the current economic climate, to either cut out routes completely or to reduce the number of flights on each route. As a result there are fewer seats available, which means that the airlines can sell nearly all of their seats at full price.
Jet2 aren't currently operating at all on that route. (Their flights start at the end of next March).
Thomson are only operating one flight per week (on Thursdays) with high fares.
Thomas Cook has these flights available:
http://i56.tinypic.com/21mbvvk.jpg
There are no other scheduled flights from Glasgow International.
However Ryanair have some cheap flights available from Prestwick. e.g. travelling out on Wed 7 September and back a week later costs a total of £211.96 for 2 people, including al taxes and charges (but excluding baggage fees and debit/credit card booking fees):
http://www.ryanair.com/en
Chris