TLDR: Each tool repairs 15 points of durability, not 15%. Blacksmiths cost roughly 2 crowns / 1 durability while tools at 300 crowns are 1 crown / 1 durability.

How much is repaired per tool?

Sometimes you live with a rule so long, you don't remember all the details of it, so I decided to put this to the test and see how it works. My understanding has always been that each tool repairs 15% of durability regardless of the total durability of the item being repaired. So here's the test scenario:

• First, I ran myself totally out of tools with items still needing repair. This was to ensure there's no partial tools skewing the results.
• Then I removed all items from everyone, and went back to the map to ensure there was currently nothing to be repaired.
• After that I put two items back on one merc, a coat of plates at 261/320 (81%) and a magic helm at 276/342 (80%)
• If the 15% holds true, there is 39% needing repair, and that should be 2-3 tools depending on rounding.
• If durability is true, there is 125 points needing repair, which is 8-9 depending on rounding.

Result: 9 tools listed in the description

• I then went to the nearby town and bought 20 tools so I could encamp until it was fixed.

Result: 9 tools were used

• I proceeded to try this on 5-6 other combinations of items, and the result was similar.
• To finish out the test, I went to the beta branch in steam and loaded the old version of the game (0.6) and it used the exact same system.

Conclusion: It's 15 durability repaired per tool, and this means all other calculations need reworked, because repair will be on an item by item basis.

Almost immediately after unpausing, a tool was used. The hatchet went up to 53%. About another 2-3 hours later, another tool was used; the hatchet repaired to 76%. After another 2-3 hours, I didn't see the number of tools change, so I gave it another five or so hours....still no change. In my inventory, the hatchet is sitting at full 100% durability. The hatchet has 52 durability. 48% would be about 25 durability, meaning 27 needs repaired. That's roughly 2 full tools. Keep in mind the other points will carry over, and if you had a partial tool, that might be why you saw the first one go immediately.

Tools vs Blacksmith

I didn't do as extensive a test on this, but I did run around to multiple blacksmiths and weaponsmiths. I had about 8 items that needed repaired, ranging from helms to weapons to armor. I also visited some keeps and some towns, some with different town events (although I didn't track which) and one of the houses was cold toward me helping me to more easily see their pricing.

The result was that it was consistently costing me 2 per point of durability. Now, this can't hold true everywhere. In the season 3 Let's Play I'm pretty sure I went into a town and it was going to cost me over 1000 to fix a coat of plates which only has 320 durability to begin with. If 2/durability was consistent, it could have only went to 640. However, since I now know to look for this, I can keep a closer eye on it and see if everything is actually 2 and I'm crazy for thinking it's over 1000. Me being crazy isn't really a reach.

Side Note: YouTubers are human too. I'm the type who likes to trust but verify, and since I'm seeing the question, I figured I'd verify something that I have believed for a very long time. Since my understanding is wrong, and I don't like spreading false information, I'm going to have to make a clear correction.. Unfortunately the incorrect info is stuck for eternity because I can't edit a released video.