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SirAntoine
Can you give some numbers to back up this calculation? Are you taking into account earning XP for looting treasure and from other sources?
"On the average, characters should go up one experience level approximately every five adventures." - Rules Encyclopedia p. 129
Using your numbers, that's 20 "good fights" per adventure. I'm not sure what is meant by "adventure." A character is not allowed to go up more than one level during an adventure.
Actually, that's about what I'd expect during an adventure (20 good fights), but I agree the term is too vague to serve as a unit of measurement.
In BECMI and AD&D 1st Edition, you earned about two-thirds of your total experience from treasure, so taking that into account, a single adventure would have 6-7 good fights and leave the rest of experience to treasure finding. The thing is, you rolled treasure randomly on tables, and while sometimes you could get tons and suddenly advance just from opening a single chest, there was no expectation of finding such a significant amount that you could ever predict when you'd go up in level. In 2nd Edition, they changed the standard to no more experience from treasure, and if you played by that you'd really be advancing slowly and relying mostly on combats.
At 1st level, the fighter needs 2,000 XP to advance, and a typical monster is worth 7, 15, or 35 experience points. The fighter later needs 250,000 XP to advance every level higher than 9th, and a typical monster is worth 650, 975, or 1,400 experience points. Maybe a few thousand the higher up you got. So even factoring in treasure, or story and indivdual XP awards (which were both small by the book), it's easily a reasonable estimate. And as I mentioned to Zardnaar, the party used to be quite large, including henchmen. The wizard alone could sometimes have a retinue of 8-10 bodyguards including zombies and the occasional golem. The ranger had a menagerie of wild animals, the fighter an entire force of armed men, etc.
It's also worth noting that even the largest treasure types, such as what you'd find in a big dragon's lair, would seldom give you more than 15,000 gp total. So if you go on a quest at high level to kill the dragon, its hoard is all very large in the story books but in practice it's not that big.
The treasure may also be hard to find by the way. If you go looking for treasure in a dungeon, you might get a little or a lot, but you have to search for secret doors, get by traps and other guards, and all the while run the risk of encountering wandering monsters. In the outdoors, where most combats might take place if your PC's travel in dangerous land a lot, you would have to find the monsters' lairs to even begin looking. Treasure just didn't fall out of the sky.
In my personal experience, 10 gp went very far. 40 minutes to an hour could be spent role playing in a shop as you haggled with the merchant. We didn't rely on treasure to get experience, but it was a sweet reward. There was nothing like the feeling of finding gold and gems, let alone magic items, too!