From the Basic D&D Companion set (1984):
When designing adventures to fit the needs of the characters, you don’t need to guess the proper amount of treasure to place; a bit of simple math will help. Use 125,000 XP as the average needed per level. If you want a group of 4 characters, all level 12-25, to advance 1 level after completing 5 successful adventures, then they will need a total of 500,000 XP to do so. They should earn about 1/5 of it (100,000) by defeating monsters and another 1/8 (62,500) by reaching their goal and performance; subtracting that, the remaining 337,500 must be from treasure. Divide that by the number of adventures (5) and you find that each adventure should bring them 55,000 gp-if they play well.
In both 1e and basic, the vast majority of XP comes from treasure. It’s mentioned in the Basic Red Book and again here: 2/3 or more of XP is from treasure. So almost by definition, an adventurer of level X has earned Y amount of money.
Still, this “wealth by adventure” advice from the Companion set, published a year before Unearthed Arcana, is reminiscent of the 3e “wealth-by-level” calculations. The reason for the advice, though, is far different. In 3e (and 4e), you need to have a certain amount of money to make sure your magical gear isn’t over- or underpowered for your level. In Basic, a massive amount of cash doesn’t make you overpowered for your level – it makes you higher level. These “wealth by adventure” guidelines are for pacing.
Basic D&D decided that one level should be 5 “adventures” long. In contrast, 4e says that one level should be 10 encounters long. I know what an “encounter” is, but I wonder how long an “adventure” is? One prepublished module? One dungeon location? One quest? I’d imagine there’s a lot of variation in exactly how much fighting there is in a single “adventure”; a game session with, say, 3 battles might count, as might a 20-encounter clearing of a dungeon. Therefore, since treasure is doled out by adventure, you could say that the vast majority (80%) of XP in the Basic game is “quest XP”: it’s given to you based on how many objectives you’ve solved, not the difficulty in doing so nor in how many enemies you killed along the way.