Cost to repair correctly is almost always a lot higher than the fence value of the material, but more importantly, repair cost is
always higher than the labor/tool cost to steal the material. Dunno how long it takes to cut off a 12 foot section of guard rail, but the fence value of that rail only has to be more than $15/hr over the time it takes to find and remove the rail to make it profitable.
Its the same thing with catalytic converters. The crackhead stealing a catalytic converter from a 2011 prius is interested in the $150-$350 of platinum in the catalytic converter, not the $2200+labor replacement cost of the thing. Considering that its ~20 minutes looking, and ~2 minutes sawing to steal the thing, we should all be so lucky as to make $150-$350 for less than 30 minutes' work.