With the cost of clothes, I feel it is best to scrimp first on the clothes no one sees you in. This excludes your spouse, of course, who will only care that you're soft and warm!
When your night gown wears out at the top, but the bottom is still OK, cut the top off, and replace it with a T-shirt. Cut the hem off the T-shirt. Put a long zipper in front to keep from pulling it over your head.
Medical scrubs make very comfortable cheap pajamas. They are available in a variety of colors and styles.
If you have kids and you know their winter pajamas won't fit them next season, cut them down for the spring/summer. Cut the pants into shorts and the shirt into short sleeves or even tanks.
My husband has several "banded collar" shirts that he no longer wears. They are still in good shape and I loved the style.
When my four kids were little, they loved to sleep in Dad's t-shirt. I went to a yard sale and found 10 T-shirts with some business logo on them for 10 cents each.