Somewhere between 250,000 GP and 125,000,000 GP. *
And it sells for full price.
Gemstones are small, lightweight, and easily secured compared to their same value in coins.
- DMG 133
The DMG does not give the weight of gems, unfortunately, but it does say that they are lightweight compared to coins, so for my calculations, I used the same weight as coins: fifty to the pound. This gives us a total of$$
500~\mathrm{pounds}~{\times}~50\frac{\mathrm{gems}}{\mathrm{pound}}~=~25,000~\mathrm{gems}.
$$
On the following page, there is a table to roll on for gems as treasure, and gives the value of each type. There's a lot of them, so they're listed at the bottom of the answer. What matters is they vary in price from 10 GP (e.g. Obsidian) to 5,000 GP (e.g. Ruby).
If your pile of gems is completely made out of Rubies, for example, you're looking at around$$
25,000~\mathrm{gems}~{\times}~5,000\frac{\mathrm{GP}}{\mathrm{gem}}~=~125,000,000~\mathrm{GP}.
$$
On the other hand, if your pile of gems is completely made out of Obsidian, you're only looking at$$
25,000~\mathrm{gems}~{\times}~10\frac{\mathrm{GP}}{\mathrm{gem}}~=~250,000~\mathrm{GP}.
$$
Don't sell it all at once, though. It's not that supply and demand is a problem, but that money is heavy, by comparison. Gem prices are static, and they are at least as efficient as Platinum by weight. It's only worth selling them when you need the money.
* Your DM might decide that you need to perform an Intelligence check to know this in character, though.
- 10 GP gems: Azurite, Banded agate, Blue quartz, Eye agate, Hematite, Lapis lazuli, Malachite, Moss agate, Obsidian, Rhodochrosite, Tiger eye and Turquoise.
- 50 GP gems: Bloodstone, Carnelian, Chalcedony, Chrysoprase, Citrine, Jasper, Moonstone, Onyx, Quartz, Sardonyx, Star Rose Quartz and Zircon.
- 100 GP gems: Amber, Amethyst, Chrysoberyl, Coral, Garnet, Jade, Jet, Pearl, Spinel and Tourmaline.
- 500 GP gems: Alexandrite, Aquamarine, Black pearl, Blue spinel, Peridot and Topaz.
- 1,000 GP gems: Black opal, Blue sapphire, Emerald, Fire opal, Opal, Star ruby, Star sapphire and Yellow sapphire.
- 5,000 GP gems: Black sapphire, Diamond, Jacinth, Ruby.
When you drink this potion your physical age is reduced by 1d6+6 years, to a minimum of 13 years.
You can't reduce something to a number lower than its stated minimum.
By a plain reading of the text, you could go ahead and roll a d6+6, get some number in the range [7,12], subtract that from 9, and the result of that calculation would lie in the range [-3,2]. Those are below the stated minimum, so are not the result. Logically, then, one of two things must happen:
(a) physical age is set to 13, the minimum allowed by the potion, or
(b) the potion has no effect.
If (a), then we have the contradictory reading that we've reduced your target's age to a number larger that it started with. I'd argue that one can't make something larger by reducing it, leaving (b) as the only possible reading.
The potion has no effect. (Unless, of course, this is not the first and the 10% aging clause is triggered.)
Best Answer
Start with vampire spawn stats instead of vampire. This has a CR of 5, so it's perfect as part of an encounter with your party if they're already low on resources from the traps, etc. Add in the shape change and Misty form traits from the vampire, which makes it harder to kill, but not harder to fight. Done.