We made this simple Tardis from a cardboard refrigerator carton and some paint. Hopefully you can make it for under $20, less if you have existing paint.
The items you'll need:
Fridge carton - Free from a large electrical appliance retailer. Larger the better.
Paint - Blue for body of the box. Rattle can black and silver for the details.
Roller or brushes.
Printer - to print out the signs and windows.
Step 1: Box and Blue Paint
Enamel gave us a nice glossy finish but took a full day to dry.