TLDR: Weapon Repair Kits are worth keeping and using
I did some quick math on whether it's worth it to keep a Weapon Repair Kit (WRK) to repair your guns or to sell it and do it yourself.
I have a 94/100 M4A1, so it would take 6 durability to repair back to full. Using the WRK to repair 6 durability of a gun would cost 3/1000 durability the WRK has, meaning that every 2 durability you repair of a gun, 1 durability of the WRK is used. Quick maths means you have 2000 durability of weapon repairs in a full Weapon Repair Kit.
The WRK (from what I can tell) always has a chance to take 0-3 of the guns maximum durability per repair, which is the same ratio as Skier so I will compare the Weapon Repair Kit's ability to repair to that of Skier's. To repair 6 durability of the same M4A1 mentioned above, I will pay Skier ₽1900, so every 1 durability Skier repairs will cost me ₽316.66, repeating.
Comparing the two numbers, and you have a ₽316.66 to 1/2000 durability, meaning you'd have to spend ₽633,320 to equate the full usage of a Weapon Repair Kit.
This isn't even taking into account that you'd be upgrading your Weapon Maintenance skill by repairing your own weapons with the Weapon Repair Kit which I'm sure would mean using less durability of the WRK for repairs, having a better chance of repairing your weapons without reducing the maximum durability, or even both.
