They regularly do and sometimes extremely substantially. They call it "dynamic pricing" which is really just a nice label to justify squeezing whatever they can out of you.
But in reality I reckon it's fair enough. We, the punters are always looking for a bargain and it's not like it's at all underhand with the old tactics a la RyanAir.
For the last few years several airlines have had a New Year sale and I've lost out a bit as I tend to book 6 months or so in advance. But sometimes the sale price is more than I paid and more often it's the unsociable hours flight times, which have little attraction for me.
But their practice is certainly quite legal and personally I reckon Monarch are one of the most professional of the outfits we fly with these days.