I am of the school that most bike thefts are opportunity thefts, i.e., someone just comes along without the intent of stealing your bike but sees it and then takes it. On tour, I take a ~10' 1/8" vinyl coated wire with a small combo lock (think a heavy duty suitcase lock) that my local hardware store makes. I also shrink wrap the cable ends so they don't scratch anything. Around $20 I would guess but mine is at least 10 years old so not sure. I can wrap both wheels around a pole, wrap multiple bikes together, or lock it to something that is something relatively far away. It weighs under a pound. For my next touring bike, I am strongly considering getting an Abus Frame Lock but may not as I would still need the cable. To me, the big plus of this one is that it is very simple to lock every time so would get me to lock the bike more often.

As others have mentioned, if they want to steal a bike, the bike is stolen regardless of the lock and chain. In my 45+ years of touring, I have not had a bike stolen while touring and only worried about it being stolen a handful of times. Unfortunately, I recently had my Co-Motion Americano stolen from my garage from an opportunity thief (garage door was open).