I guess the M&S option is probably the cheapest. If you're really experiencing problems, and don't want to go Lycra (in line with jonny jeez, you should really consider that), then I'd go for the cycling undershorts that have been designed specifically to your problem. Craft have nice ones, Rogelli does, and the Endura kit is good as mcshroom says (they do a superb one, the MT500 protector, but that comes with a bit of a price tag..).
