Forewords: it's been quite a bit since my last DM'ing (20 years at least), and I don't have my boxed sets at hand reach right now, so I'm going a bit by memory.
First of all the answer to the explicit question: as already stated by Chemus each gp is an XP point too. But, do take notice: no more than one level advancement for each adventure! No matter how billion gold pieces equivalent you can bring home, after a bit is "just money".
And it's an X module. That is, the players have already got their fare share of loot in other adventures, they all probably already have a tons of magical items, +2 swords are a dime a dozen, and so on. It's listed as level 3 to 6, but X officially begin at level 4 and unless your party is composed of 8+ peoples this module will be played mostly with level 5 characters, that means that xp needed for advancement will begin to be quite high.
Second, a bit of unneeded and unrequested "bigger picture".
The most important fact about this module is its background. See, the Amber Castle is not exactly meant to be played as just a stand alone module...hell, ok, it is, but what I mean is that it's part of a huge, bigger background and it fits into that background: Glantri.
Ok, let's make things clear first: Glantri settings will not be finalized 'till a lot later, 6 years, when the GAZ3 will be published. Glantri as a setting already existed (it is mentioned into the module itself, too), but I was 4 year old when X2 was published and I wasn't much in D&D back then :-D so I have no idea of how much it was detailed. So what follows is, in a strange and interesting twist, "Why the X2 was designed that way, if time in the '80s would have flown in reverse". Or, to be more serious, how X2 retroactively fit into the Glantri settings and what could be the best way to play it.
Glantri is a nation inhabited by wizards, ruled by wizards, and with magic everywhere. It's a rich nation, full of really healthy wizard's families (basically whatever Rowlings wrote comes from here, if you delve deep), and one of the most powerful family of all -if not the most powerful- is the D'Amberville, with one of them (Etienne, a tall, thin, old powerful wizard with a long white beard and which is the headmaster of the wizarding school) being an immortal and at least two or three other family members on the path to become one, too.
Your players have spent their B levels (mostly) underground, reaching level 4 and witnessing endless marvels. They thought to be uberpowerful, to have seen everything, they feel ready to take over the world. That is the moment when the DM bring on the table the X rules set, its hexagons, an entire planet, and bring the players back to reality. And to amusement.
If the DM follow the flow, so to say, their first new experience will probably be running away from a T-Rex and understanding that the B was just an appetizer (which, by chance, is the same thought the T-Rex had when seeing the characters...)
Your players wander around a bit, gain one or two levels, and somehow ends up heading for Glantri...
...which is basically wonderland, only scaled up quite a bit:
- Magic at every corner? Check.
- Electricity, washing machines, appliances, water boilers? Check.
- Motor boats? Check.
- Azkaban? Check.
- Magic creatures serving at tables in pubs? Check.
- Alleys full of magic shops? Check.
- Children with magic power out of control, capable of transforming by mistake entire parties into flowers pot? Check.
- Dark wizards? Hordes. Check.
Squibs, muggles, animagi...errr...I mean, secrets and drama about children born without magical powers, some occasional racism against not magical able beings, secret shape shifters, and so on? Check.
- Chernobyl leftovers? Check :-D
- Magic schools for wizard's children? Check
In short: every possible source of amusement and weirdness and you can think about? Check!
So in the end the players are moving to Glantri and along the way they end up into the ancestral home of the aforementioned D'Amberville, hugely powerful and rich. In this sense it fits perfectly that the module is full of wealth and magic objects (and weirdness, too), and given the fact that the players will be able to just scratch the surface of it is even more interesting as they will have to decide at every step what to leave and what to keep.
Even more, it makes much more sense (and fun) if you join all the dots together and use the X2 to tie the players to a bigger campaign in Glantri. Have them reach Glantri and spend a few weeks in the city trying to make sense of all the stuff, getting used to names and families and such. Have them leave the country for a stupid generic commission, and then stumble inside the Amber Castle. They now have some generic hook into the castle background, they wander into it trying to understand what's going on, and when back to Glantri they will have some reason to start investigate the D'Amberville family. And/or maybe the castle was supposed to be sealed in some way, and Etienne himself will begin to keep an eye on the party because the seal was opened in some strange way, maybe by some other member of the Circle of Rad...