Some themes derive from nature and have items that are easier to come by. A Scandinavian theme or a Classic Christmas theme might be good options for a budget minded Christmas.
To make it even more budget friendly, look at the decor you already have and find a new theme that will allow you to re-use some of those items.
I started shopping, planning and making well before Halloween. When going for a new look, I prefer to spend November creating and shopping for decor, and then December can be all about my family and getting gifts.
Make a pinterest board with your Christmas theme and fill it with inspirational images. Make a shopping list of what you are looking for, and stick to it.
I’m preaching to myself here, but I realize the importance of not depriving my children of memories and traditions, in spite of changing Holiday decor.
Our treasured countdown made from Grandma, the evening story books, The Nativity passed down from relatives, the Fisher Price Christmas toys . . . those will all come out in the right time and for the right place, regardless if they match my decor.