The most budget friendly sign material similar to a corrugated cardboard but is all plastic. The standard 4mm material is rated for up to 2 years durability. Rigidity depends on the thickness. Generally, most 4mm  coroplast signs are around $7 per square foot including graphics. Quotes are provided for larger quantities and special orders.  Laminated signs are more. 10 mm and 6 mm are also available but subject to special order fees since I don't normally stock those thicknesses.  4mm Coroplast also comes in several colors besides white. I try to keep some yellow, black, and orange in stock. EDIT: I keep 10mm coroplast in stock now as it seems to be quite popular

Aluminum Composite Panel

This material gives your sign a more professional look, it is smooth and rigid and stronger than coroplast.  It consists of a layer of plastic sandwiched by 2 layers of thin aluminum. The durability rating ranges from 3 to 5 years.  The cost depends on the durability rating of the ACP and vinyl graphics and ranges from $12 to $15 per square foot. It can be double sided.

Plexi Glass

It's a nice elegant sign material and can be backlit.   It comes in white, clear, bronze and a few other colors. (white is shown) The durability is superb and comes in unbreakable. The cost is always custom quoted. 

Painted wood (Crezone)

durability is from 2 - 5 years.  Very rigid and comes in 1/2" or 3/4" thickness.  Can be double sided.  

Aluminum

The most durable of all. Larger signs 4'x8' will need support.  I use powder coated aluminum blanks with rounded corners for smaller signs.They also come in several thicknesses.  please ask for a custom quote.

Coroplast

Poly Styrene

It's a thin or thick, smooth plastic that comes in several thicknesses, can be rolled up but is not suitable for long lasting outdoor signs.  It warps in the sun.  It's great for indoors and can be contour cut to almost any shape.  Great for temporary signs.