hi gros21,
i only buy cheap ones now as my 2 little gremlin girls seem to have a knack of breaking them for me but i found the cheapest reasonable polarised ones are on sale at bcf, supercheap etc. for $30. at that price when i go thru 2 or 3 a year it dont matter!
they do get scratched but only when i wipe the salt off dry.
cheers ryan