It a GM call right now. there been a few post out there that people have clicked FAQ on trying to get answers but none to be found yet. because people are not sure what the DC should be either should it be 20 because it is mithril and it is considered a master work material. Or do you do the item's DC, then the 20 later. just like the mounted combat rules the crafting rules are a mess, I think they have been cleaned up in unchained a little, bit. you may want to use those rules, look them over.
Did they not take a 10? by the time you can afford to make something out of mithril they should be able to take 10 and auto pass the dc 20?? you can almost do it at level 1 1 rank +3 tranined, +2 master work set of tools + int bonus most people have. Not to mention any race bonus or assistance helping via aid other action? There even a spell as wizard could cast that is 1st level that gives +5 to the check. could easily put a crafting at well over a 20 with take 10 at level 1, so failing should be impossible.
then you run into the other problem with crafting something out of mithril, it take for ever to make anything out of it due to it's cost and the crafting rules. the only really effective way to make something out of mithril in a decent time frame is via fabricate spell which is 5th level or with the magic item amazing tools of manufacture. which cost 12,000 gp in them self. This should give you an idea of when a players should be getting access to mithril gear. which is often around 7th to 10 level by then.
If you are the GM, and they are around 7th level, and depending on what situation, you may want to be little lenient on your player considering how messed up the rules are. Is he aware he could take 10, is aware the npc or another PC could have aid other on him or cast 0 level spell guidance on him as option. Are you as GM aware he could take 10 on this check?
Now would a take 10 even pass the check for him. If he did not have any hardly any ranks in craft and could not take 10 and pass and had to roll and knows about aid other, spells ect. I think he lose half of the potential work he could have done on a successful check.
