ok, I can pay a Smith to rep it for coins, but this bow is current non-repairable on an anvil, because it requires more string than it can take into the material slot... definitely a bug, and bow repair requiring nearly a full stack of string is just CRAZY anyway.
This is just a standard Rivendell Bow with some lame mods.
I think this is an intentional feature to prevent you from using the same equipment forever. Each time your repair an item it adds a little to the repair cost until at one point you won't be able to repair it anymore, at least not on an anvil.
the problem is that every mod you add on a bow raises the repair cost, so in a way putting mods on that make them last longer is raising repair costs by a large margin - nearly making the mod useless. Sure you can shoot them longer before repair, but when you repair them you pay a lot more material cost also. Stupid design choice. This bow is simply too damaged (but only like 40% or so really) and with the mods on the bow the cost goes over 64. It's a design bug.