From my experience, the classic line such as the Saffiano, Tessuto and Calf leather doesn't go on sale at Prada... depending how popular a seasonal item is, it may or may not go on sale.
From my knowledge, as what terri_berri said, the classic line doesnt go on sale but the seasonal ones do. The last sale in the Chadstone boutique was in April (if I do remember it correctly. I remember seeing it during Easter).
thanks il take a look at the classic lines