With a longsword as a luxury weapon, 15gp = $1500 looks pretty reasonable to me. 3e D&D has a bit of a problem with inflated armour costs, 1500gp = $150,000 for a suit of full plate is possibly rather excessive; some 16th-17th century plate armour worn by the high nobility may have approached that sort of figure, but in a D&D world those guys would be wearing magic armour.
4e D&D lowered the cost of plate armour massively, to 50gp - similar to Basic D&D's 60gp. This was done for purposes of playability rather than simulation, but if that is for a very basic breastplate & helm (3e 'breastplate') with the +n/magic versions equating to more complete and ornate suits, then it actually looks fairly plausible. At 50gp = $5000 that's still a fair whack of change. In 4e 1500gp/$150,000 gets you +2 magic plate armour.
The problem being is that we're not looking at 16th and 17th century style armors; rather, we are looking at armors from the 14, 15, and perhaps EARLY 16th centuries. Plate Armor really comes into its own in the 15th century and through to the 16th, but comes from so many sources during the period of the transition. Cheap munitions armor in plate came about in the 16th-17th century as firearms begin their dominance, but early plate armor? That stuff was expensive.
Specially fitted pieces of high-tempered, high-quality steel made to the wearer. It also includes all the accouterments (arming jacket, possible plated surcoat instead, tabard) that must be tailored. If you figure the labor spent on forging plates, fitting them to the wearer, confirming fits, preparations, and actually fitting it all together without the handy tools available to modern craftsmen could quite easily take a year.
The whole thing is that the armor IS Special. Plate Armor was a specialized armor, almost Exotic in its presentation and service when compared to similar solutions for the same problem (jack o' plates, plate "mail"). Plate Armor is actually assisted by the craft rules to prevent it from taking as long as it COULD to complete.
Plate requires at least DC: 18 to craft. A normal 1st level smith (as presented above) could make plate in about a year, which a few texts point out (based on orders from armorsmiths) is about the right amount of time it would take (~ 50 weeks). If you carry your Apprentices along for their Aid bonuses as in the above examples you can improve your lot quite a bit via Fast Crafting (increase DC to 28, 29 overall check with +10 Aid bonus, 81.2 gp/week improvement)... Your Plate Mail would be produced in about four and a half months. Two suits a year along with a pair of masterwork longswords and various odds and ends can make for an intense but profitable year. Since you're spending 81.2/week you could technically focus on making your two plate suits Masterwork as that 'odd and end'.
End of Year you have 2 Masterwork Plate Mails and 2 Masterwork Longswords for your troubles; overall cost: 1675 GP up front. Profit: 2255 GP.
Royal Armorers live QUITE well

It is definitely possible to have a single 1st or 2nd level Expert leave behind a legacy that can move his family up the ladder through his frugality. Living in a Common lifestyle and reinvesting into maintenance for his forge and wages (10% on maintenance seems to be about the 'round number' breaking point for the idea) can have 1500+ GP to work with.
If he uses this money to expand his workshop, pick up more apprentices and journeymen, and build himself up he could develop a small empire within his own service. If he can invest into various businesses he can produce a supply of income. An Inn runs 13000 GP to be built from the ground up, and would probably sell for less than that. With twenty rooms, ten stables, and a tavern a common inn can produce 2737.5 GP/year at 50% capacity for the year. If you figure a heavy loan of 10% interest with half down and half of its overall cash reinvested the inn can be fully paid for in a little under 8 years (I factored in the costs of everything too, yes

There is plenty of money out there for those who are skilled in the D&D world, and a merchant could quite easily start a humble smithy.
Slainte,
-Loonook.