How are they being stolen?
Most likely bolt cutters (https://www.welovecycling.com/wide/2016/05/18/former-thief-interviewed-bike-stealing-actually-work) or increasingly a cutting wheel and angle grinder
I got 12mm cable locks (Lumintrail Bike Cable Lock, Self Coiling 12mm Braided Steel Cable Resettable Combination Cable Lock with Included Mounting Bracket - 2-Pack) under $20 for two) for the kids but that is mainly just for school.. It seems that there is no lock that a experienced thief cannot quickly cut, and that the idea is to make it hard for them, while GPS tracking is highly recommended.