Never used a magnet before. *checks* huh. They count as bait, not a lure. That is horribly expensive. You can make them out of iron bars, but they don't seem all that useful when compared to the cost. Apperently, magnets don't increase the speed of catching fish either. Correct me if I'm wrong on that. Wiki says you only double your chance to get a treasure chest. Well, if I double my chance to get a chest (normally 15%, and becomes 30%), but take twice as long to catch a fish, then this bait is worse than useless.
I'm not for lowering the cost. That said, if I have to spend an iron bar to make 3 of these things, or spend 1k gold to buy one, I would like at least a 80%+ chance to get a chest, and I think there is a fair argument for this to be 100%. A lot of chests are pretty useless, after all.