Murrdox said:
Anything else is the DM allowing some munchkin.
There is no "some munchkin", either you are munchkin or you aren't. And as it stands, a little resistance (5, maybe 10) against a single energy type isn't even much powergaming, especially since he's willing to pay for it.
As for the pay, maybe treat it as a special material, just like adamantine or mithral, with similar market price modifiers and progressive effekts:
Light armor: Energy Resistance 5; Market Price Modifier +2000 gp
Medium: 10; +5000 gp
Heavy: 15; +10000gp
Providing the special material will half the price you have to pay, the rest being normal materials like leather or metal, and the wages for the smith.
Lord Ravinous said:
And they'll have to keep earning it even after they have the armor, cause, if they run into another dragon of that type, they'll be dealin' w/ a VERY pissed-off dragon, that just might be older than the last one.
That's to true: in a game I play in, the party druid had an armor made of dragon scales (It was some sort of quest item: green dragon scales, gave him acid resistance. We didn't encounter that much acid, but on those few occasions, it was OK.) Later, we stumbled into the lair of an old blue dragon (we thought it was the hideout of a sorcerer, but that one turned out to be eaten by the current inhabitant). We were lucky, though, for he didn't want to kill us, we "just" had to pay for our lives and our information with magic items.
Anyway, he asked the druid whether that's dragon scales he's wearing, and he answered: "That? No, that's crocodile". We were very lucky that day, for the dragon ate that story. Instead of us....