Away three months or so, when it’s cheaper (3,6)
I believe the answer is:
off season
'when it's cheaper' is the definition.
(when holidays are cheaper)
'away three months or so' is the wordplay.
'away' becomes 'off' (synonyms).
'three months or so' becomes 'season' (each season lasts about three months).
'off'+'season'='OFF-SEASON'
(Other definitions for off season that I've seen before include "Time of year when usual activity or business is slack" , "Time of year when business is slackest" , "less popular period" , "In less busy time" , "slack period" .)