Some of the mbig netal merchants like Smiths can knock at least 40% off Aluminiumwarehouse prices, though their cutting to size is not nearly so accurate in my experience! Don't forget that a bit of thought and ingenuity in the design can save you weight of metal and hence money - for instance a side plate of 10mm ali can be made to be stiffer than a piece of 20mm by adding some vertical stiffening ribs