I've been mulling this over for a while; after all, this is the entire point of why the characters would risk life and limb going into dark ruins of evil cults in the first place. Otherwise, they'd settle down into the nice safe positions of village blacksmith/publican/prostitute or whatever. So it had better be worthwhile.
Also, it's there for the enjoyment of the players (including myself), so it had better be compelling, or at least momentarily interesting. No, "you find $4000 worth of stuff," or "you find a thousand silver pieces" in my game. Even if the players don't care about the palladium tiara with a frog motif and just care how much it costs, it will amuse me to put it there in the first place.
Peter over at Dungeon Fantastic has a helpful post about what he's learned about treasure from running Keep on the Borderlands and his megadungeon (play reports of which are what got me doing this in the first place, because they're awesome). I never really felt the temptation he mentions of giving too little treasure, because I have ways to soak up and control money written directly into the adventure (training costs, plentiful hirelings at least after the moathouse, carousing, potential for theft, plentiful mundane/consumable items while keeping enchantment rare by forcing people to make a long potentially dangerous trek to the city and then wait for the enchanter, even animals), so I don't feel worried about treasure destabilizing things. If making a big score means the PCs can replenish their supplies, upgrade their gear, and hire a bunch of people to watch their horses/hold their torches/fill out the front line, well then, good - there are a couple places the PCs pretty much need an army. Nevertheless, the post is useful because it puts the practical experience of someone who has done this sort of thing before in one place with reasons and explanation.
Also, it's nice to know that his experience syncs pretty well with what I've been thinking.
Generally, for every gold piece worth of treasure written into the original module, I will substitute $5 worth of GURPS treasure. If this is not in a hoard of some sort (e.g. carried by monsters), then it will probably mostly remain coin. So, for example, one monster carries 1-6 sp, 1-6 ep and 1-6 gp. We'll assume I rolled 4 for each, which would translate into 6.25 gp. This means he'll carry $31, or four silver and fifteen copper, using the default values from DF 2. It could be any coin that adds up to $31.
For gems, I'll convert those by rolling randomly on the random gem creation tables in Dungeon Fantasy 8 - Treasure Tables - which, seriously, if you want to run (or even just play) DF, you should get. It's probably the most awesome book in the series. In fact, I like gems, and will probably be expanding the gem creation table to include a number more, like zircon, that exist in D&D but not the table. #Then I'll do the spot-check above to make sure it equals or exceeds my 1 gp = $5 base.
For hoards, I'll follow the same guideline for establishing total value. However, for specific non-monetary treasure, I'll convert it according to the DF Treasure Tables, picking items and embellishments that best mimic the item as given. For example, in the moathouse there is an ivory box worth 50 gp. After conversion that's a small stone box made of Fine Material (ivory) and Minimal Painting/Enamel. Since I'm feeling pedantic, I'll roll on the Decorative Motif table and find out it has a leopard motif. If I were feeling especially so, I might give it more embellishments and then make it damaged by its treatment, reducing its monetary value back toward the desired amount.
I now have an ivory box about the size of a large book painted with cavorting leopards, worth $250 and weighing 4 lbs, reduced from 6lbs because ivory shouldn't be as heavy as stone.
Once I'm done converting specific items, I'll take half the value in coin, double it, and then roll in the treasure tables (whichever ones I choose, depending on monster, placement, etc.) to pick out various valuable objects to substitute. I'm doubling the halved amount because DF gives 40% for sold items that aren't jewelry or gems, unless someone takes wealth as an advantage, and such objects are invariably harder to carry about than coin anyhow. So instead of a cache of $10,000 in coin, the PCs might come across $5000 in coin and $5000 in assorted fine garments, spices, tea services, books, and dungeon-delving gear.
Then, on top of this I'm going to roll once completely randomly on the treasure tables, just for kicks. Sure, this could end up with a pair of gnolls guarding a giant magical cannon, but if that happens I have an interesting story on my hands. Maybe nobody realizes what it is, because its in several pieces, or these gnolls managed to avoid having it taken from them by the giants next door by threatening to use it, being comfortably affluent roguish adventurers who found it and are waiting for a buyer to get back to them. (By the way, would the PCs like it? Only 5000 gold pieces and three hirelings and it can be yours!)
In the moathouse, this doesn't stand up very well. For example, one treasure hoard contains the wonderful sum of 2000 copper pieces. That's 10gp, for those playing along at home. That's just going to be kept as 2000 copper pieces, or $2000 in GURPS terms. (It's thematically appropriate, too.)