From the FAQ at the end of the Wonder Pack rules:
If multiple players take cards from the discard pile during a given turn, the resolution order is the following: Halikarnassós, The Great Wall, Manneken Pis, Salomon and finally, the Courtesan’s Guild.
My guess is that nobody thought too hard about balance here: it's important for there to be some order, but it's rare enough and minor enough that it barely matters which. Instead, note that a copy effect always comes after the thing it's copying: Manneken Pis after the other wonders, and Courtesan's Guild after the leader. I'd speculate that this was designed for feel rather than for balance - some players might think it unfair or unintuitive if an opponent could copy their effect before they'd gotten to use it themselves.
Continuing with the speculative post hoc justifications: imagine a game with one of the wonders in play and one of the cards. The person holding the card can see the wonder. The person with the wonder, however, might not even know there's a conflict since they might not know that the card is in play, so it would be quite reasonable for them to build the wonder stage and then grab the discard pile immediately and start looking through it. Rather than make this behavior wrong, the rule just codifies it as the canonical order: wonders before cards.