@Nitronic0 You 're really trying attenuate the context in order to make it seem like a huge event. First of all, how much do you (and not your family) make? Mc looks in his mid twenties and makes around 40k annually working as a civil worker. Can you relate to any that?
Second, they are not rocks but crystals. if you found a dozen of huge crystals in a cave irl, you'd expect that to make some good cash out of it. Moreover she told him they are rare. If you have two functioning neurons as an adult you'd 150% associate them to precious stones (like diamond). So in the end, there should not be any huge surprise. Poof is, the two girls are acting like adults and do not freak because of money and he acts like a kid in caparison. Be it 6 months or 3 years of salary, no working adult would freak out because of "too much" money, simply because, if you already worked your ass off for years to get a salary, you'd know that it's always useful and you'd have projects to spend all of it in 5 seconds. Like i said, being surprised and happy is normal, but losing it over 15k is an 8 yo reaction.
Lastly, even in poor counties, 15k is not a lot, you can't buy a huge house nor a European car.