This is from Gayle's Preschool Rainbow website. It offers some ideas for inexpensive block props.

Cheap Block Play
Materials: Empty boxes - from small shoe boxes to big boxes, such as the ones diapers come in and retail outlet boxes.
Description: Put boxes in the block area where there is room for building and exploring.
Comments: Children love playing with the different shapes and sizes.
Block Time Prop
Materials: Small green containers that strawberries come in.
Comments: Have all of the children save the containers from home.