So I just broke a rank 4 blacksmith thrall and attached him to my bench and had a realisation. The material cost reduction he gives now makes it cheaper to make a brand new sword than to repair my old one from none to full. I use the ancient khopesh which at base costs 20 steel to make, to repair from no durability to full costs 15 steel but with a rank 4 blacksmith thrall it costs 10 steel to make.
Suggestion repairing at a bench should be cheaper than repairing out in the open, bench repairing further reduced by your attached thrall, to avoid this odd situation of it being cheaper to make brand new items.
And just an idea on the numbers, currently it seems that items cost 75% of the base material to repair from none to full, repairing at the bench could cost 50% of the base material cost, with each level of thrall reducing it a further 5%
Example being ancient khopesh,
field repair cost, 75% 15 steel
bench repair cost 50% 10 steel
bench with thrall 1 45% 9 steel
thrall 2 40% 8 steel
thrall 3 35% 7 steel
thrall 4 30% 6 steel