In reply to cander:
Don't forget to delete any cookies before looking up a given connection for the second time on websites like expedia, or even consider using two different computers for cross-checking different portals.
Apparently, a second search for the same flight indicates to them that you have already made up your mind and will fly, so they assume that you will also book at a more expensive price.
Reading about this in the Sueddeutsche Zeitung saved me about two hundred euros for a single flight to Toulouse just last month: First check, cheap price on offer, but limited availability, second check after discussing details with daughter, cheap offer gone, third offer after deleting cookies, cheap offer magically available again...
Christian