There are a lot of ways to decorate at Christmas time without spending all the money "they" think you should. Here are some of my favorites:
A bowl or basket of pine cones and small ornaments
Leftover, old garland wound through every day decorations like figurines and pictures.
Candles in the appropriate color. (You can buy them cheap right now and even cheaper after Christmas.)
Old Christmas pictures, displayed in hand made frames and brightened with small tufts of (fake) evergreen or holly berries.
Red and green anything: I bring out the afghans, the rugs, the vases and bowls and cups. It all says Christmas when the season is right.
There are more ways, of course. There's a thread in the Dollar Stretcher Community about frugal Christmas decorations that has some good ideas. There's also a good piece at Organized Christmas.
One thing I almost forgot - newspapers and magazines are full of Christmas pictures right now. Find a theme, maybe Santa Claus, or stars for instance, and cut each figure out then paste them onto pasteboard, filling it completely. Then cover the whole thing with a thin layer of white glue and set it on a piano or display it on a wall. You could even cut a silhouette of a tree and glue cut out ornaments. Now, that's frugal!