I almost hate to respond, because I admire anyone who gets published and you've obviously put alot of work into the system but at least for my purposes that's a critical fumble. You are clearly more internally consistant than the base system, but not so much so that that you would represent alot less work for me.
1) The chair. A 23 in a craft skill check represents a fairly skilled Craft (Joiner) check. Assuming a 10 was thrown on average, that's +2 for Int 14, +2 for skill focus (Craft (Joiner)), +2 for having access to masterwork tools, and 7 ranks in the skill - incating a 4th level character (expert or commoner). This is an extremely skilled individual. And yet he spends 5 weeks total in the manufacture of the chair. Even if we are assuming that he starts with raw wood this is too long. Perhaps a stone age craftsman with stone tools maybe, but not for a craftsman of the assumed (psuedo-medieval) period. A good joiner could turn out a simple chair (or chest) from planks every 2 days or so. We know this partly because modern heritage craftsman can do about that, and because we know that a peasants had these furnishings (at least in small numbers).
Assuming a gold peice standard for wages - which is unhistorical but true to 3rd Ed. D&D - a simple chair should probably not cost more than 4 or 5 days wages for an unskilled worker and two days labor for the craftsman that produced it. In otherwords, the answer I'm looking for is less than a week of work (2-6 days) and about twice number of days in coinage as a final selling price (4-12 gp). If a simple chair costs about two months wages for an unskilled craftsman, no one could ever afford one. On the good side, your ratio is pretty good (35 days to 60 coins selling price) but the scale is way off IMO.
To a certain extent, I would hope that you realize that the price on the chair is too high. I mean really, do you want players carting off simple wooden chairs as treasure because thier appraisal indicates that a 12 lb wooden chair is worth about 6 lbs of gold?
2) The tiara. In this case, the problem is the opposite. The tiara is an example of a raw good whose fixed value is known by definition. So everything in the difference of the final price and the tiara price has to be added value. Even if we factor in some charcoal to the cost of raw materials, we are still going to be left with something around 10 gp worth of goods. Yet the final value you come up with of 7 days labor and 210 g.p. value suggests that the value of the labor of the goldsmith is about 29 g.p. per day. This answer is just too high. While its true that goldsmiths were very wealthy men, mostly this is true because they were money lenders and served as proto-banks. Thier labor probably wasn't worth more than a couple of times more than an average skilled laborer - lets say 6-10 g.p. per day. That yields us a final value of around 50-80 gp. Again, your answer is not close enough to expectation.
3) The plate: Your answer is more reasonable with respect to time and cost, but on the far end of the low side of reasonable on cost. Plate armor could be worth even undecorated 1200 or more days wages. The 1500 gp in the book is actually one of the few prices in the price list I consider reasonable (probably because armor is one of the few things that is easy to obtain historic prices on). I think you are probably underestimating either the value of the labor of an armorsmith, or the value of steel, or the difficulty of working with steel using feudal techniques or something.
The system is evidently simple enough to use, but doesn't provide numbers close enough to what I'm looking for. Believe it or not, inconsistancies in the pricing in D&D is not merely a matter of petty adherance to realism for me. I've already been in one campaign where Gygax's mixture of realistic (simulationist) wages for labor and unrealistic (gamist) prices for PC goods (and rewards) seriously distorted the campaign when wealthy PC's began to realize that they could leverage enormous ammounts of labor quite cheaply. Since that time, I've often wondered to myself whether the safest approach to something like the Tomb of Horrors (the original) was to bring in a few hundred laborers and painstakingly tear the thing apart. Because Ascerak in the orginal is not a responsive villian, unless the game master metagamed to stop it, such a plan would likely result in much of Ascerak's tomb be carted away for resale. This leads to heroes more in the mold of Belloq than Indiana Jones. So for games that move beyond hack and slash, this is to me not a trivial issue and a really good answer would be of great value to me.
As for the gemstones, its kinda wierd because Gygax clearly knew alot about medieval enconomies, but his prices for gemstones clearly indicate that he thought of them as gamist awards because using a silver peice standard of wages, they are probably 50 times higher than they should be. That doesn't present a real problem (I can easily just adjust prices or raw materials), but if the system itself is producing big inconsistancies elsewhere.
Ultimately, I don't blame you so much. D&D economies have been messed up since the beginning when Gygax decided (for expressly gamist reasons) to assume that the prices in the price guide represented (and would represent whereever the PC's went) the sort of hyperinflated prices encountered in the Klondike during the gold rush rather than ordinary prices. This is all well and good if you assume that PC's are walking money banks and towns are temporary rest stops between dungeon crawling, but when you try to move beyond those assumptions the original price lists (and everything that has happened since then) kicks you in the butt hard.
