In reply to Tony the Blade:
You may have noticed that no one (or virtually no one) uses "seasons" anymore because it's hopelessly vague. I have a couple of tiny synthetic bags, probably about 600 grams, bought from Tesco, a decade or so ago and cost a tenner each. They're not super warm but I've been fine in them in UK summer conditions - mummy shaped so pulling the hood up and tightening the opening works to make them feel warmer. If it's not that cold where you are going something like that might work - Aldi has sold similar in recent years.
You won't find many down bags for under a 100 quid, but Decathlon might have something. Not sure how cheap the cheapest Alpkit ones are these days, but worth looking there - but this years £100 is three years ago's £75 and you didn't see many down bags that cheap back then.
It's the old inconsistent triad of sleeping bags again I'm afraid. Cheap, light, warm - pick two.