Should I just call it 5Economy?
In any event, after being called out on my math, I settled down to think about the world-building ramifications of the crafting rules a bit more.
The basic math leads me to some conclusions which - fortunately I think - replicate (in a rule-of-thumb, simplified for gaming way) the sort of conditions you'd expect in a faux-medieval economy.
1) Limited Supply
This is the really big takeaway. If, as a craftsman, you have to pay 50% of the cost of an item upfront in materials (or even 25%, really) the item represents a loss to you personally until it sells. The bigger the ticket item, the more you'd have to plowed into it. A suit of plate mail represents 2 YEARS of living expenses for a tradesperson in materials...
Plate mail is a bad example here, because it is, by it's nature, a very limited-demand item, but even something like a longsword (15 gp) represents a lost week's wages (at 1 gp/day) until it sells. So only a well-established blacksmith can keep much inventory - it costs too much.
It also means that local blacksmiths, who presumably make their money just "practicing the craft" and maintaining a modest income, don't get many opportunities to sell big-ticket items like this, so they are unlikely to have much stock. Sure, they'll make it - but only if somebody wants it.
The upshot here is that it doesn't make much sense to be able to get expensive equipment in small towns or villages unless there is some other reason for it to be available. Sure, you can get it made, but there won't be much/any lying around.
2) Pay up front and expect to wait.
If it costs a smith 50% of the finished good (or even 25%) to make something, they are gonna want a deposit. Probably for the entire materials cost. Unless you're a local and he knows where you live and whatnot. Transient murderhobos get no credit.
If the smith is solo, divide the cost by 10 to figure out how many days it will take. If he is part of a team, multiply the team by 10, then divide by that. Minimum 1 day.
Example: Longsword - 15 gp. Cost you 7 gp up front (15/10) - wait a day and a half.
3-man team for Plate Mail - 1500 gp - cost you 750 gp up front (1500/30) - wait about 2 months (50 days)
This math would break down differently if things are less than a gold piece, I think. But unless the total cost of all the items exceeds 10 gp - the answer is "one day".
3) Some parts need houseruling/handwaving.
Darts. They cost 5 cp, or 1/20th of a gp. If you can make 5 gp worth of materials/day, then you can make 100 darts a day. Again, if you are paying 50% of the cost as materials, maybe you are just gluing the fins on and screwing on the pointy bit? Still seems fast.
There is some weirdness in the sense that a gold dart would presumably take much longer to make than a normal one, that sort of thing can mostly be handwaved (or not really worried about)
4) Any smith can forge anything.
There is only one "smith tools" set/skill, so technically speaking any smith can forge any item. The materials cost is clearly a limitation, but presumably not if you are putting down a deposit. Realistically (or as much realism as possible in elfgames) you might want to require larger cities for smiths with the skills to make highly specialized/expensive items, like spyglasses or plate armor. Even swords (heck, especially swords) had very specialized skills associated with making them.
5) Scale up for Art Objects.
Time/cost is determined by end value. If you want to make a really beautiful, valuable item, just pick how much you want it to be worth (or ask the DM), pay the materials cost (meteoric steel is pricey) and get to forging.
Hopefully the DMG will have some magic-item creation rules, but until then, requiring that you start with a very valuable item is pretty common.
Obviously, PC's won't be looking at using crafting to make money, but a PC (or ex-PC) with some capital, contacts in the sword-swinging community and knowledge of blacksmithing has a pretty good retirement plan available.
So the house rules I'm going to start with on crafting are "Pay anywhere from 10% to 50% for materials" and "You can make an item a work of art by paying more in materials/crafting time".
That's all for smithing, although it looks like performance might need a look, too.