I unfortunately have to agree with folks on the wood vs. 8020. Wood has less than 1/3 the stiffness of aluminum, and MDF is probably more like 1/5. While you can overcome this with geometry (for example, everywhere the aluminum is 1.5" thick, make the wood 3" thick -- resistance to deflection is a function of material stiffness * thickness^3), it is difficult, and will result in a heavy gantry and z. You could potentially get away with making your longest axis out of MDF, however. This is where the majority of the aluminum goes into machines.
Another way to save money with extrusion is to use carriage bolts turned upside down in the t-slots rather than 8020's expensive fasteners. It's just as strong (if not stronger), and 5/16-18 carriage bolts cost maybe 1/8 of the 8020 t-nut/button head solution.
Ahren
www.cncrouterparts.com