Thats probably the best idea, using really cheap bags. But then what do I do with my snazzy nice bags? Get a snazzy nice bike to go with them, and go and long winding tours through southern Arizona and Mexico. I'll explain to my wife it's because of panniers I'm going away for 2 months. yeah thats the ticket!
Your two beat options at todays day in age is to either disguise them as boring potato chips, but them you would have to always leave some good chips nearby. The old little bag of quality beats big bag of boring trick. Or go buy a baby ruth every once in a while and just smear it everywhere on the flaps, remember to have a few peanuts still lodged here and there so that even at a semi close look the thief will be pretty sure its poop on there. Everyone is scared of poop, except moms. But they dont have the kinda time to steal panniers... They have kids. Real poop works even better