This model of venus was made out of a styrofoam ball. Foam balls are less than $5 at AC Moore at Assembly Square.
This model of Earth was made out of play dough. You could also use clay. It can be small, if it's detailed.
This model of Earth was made by coloring on a coffee filter with blue and green markers. The coffee filter was then brushed with water using a wet paper towel. After it dried, it was glued to a piece of black construction paper. It costs less than $5 total including the markers, coffee filter, black paper, and star stickers.
This picture of Saturn was drawn on black paper using color pastels. You can spray it with hair spray after to keep the pastels from smudging.
You can cut up tiny squares of construction paper and glue the squares onto a paper plate to make a mosaic of your planet. If you need construction paper or a glue stick, just ask Ms. Coburn!
How to Make A Planet Out of Paper Mache (Videos in English, Portuguese, and Spanish):
In English:
In Portuguese:
In Spanish: