I don't think the money system in game is supposed to be represent US dollars directly. There's no way that lunch with Maya was $125. I mean, of course it's possible to spend $125 eating lunch for two people, but not without a lot of food, some wine, or both. And $250 at the school cafeteria? University is crazy expensive in the US, but not that crazy (not yet, anyway).
Replacing (virtually) all the windows at the mansion, however, could easily cost thousands of dollars. Especially if they are modern, multi-pane windows custom built and fitted, those can easily be a few hundred dollars per window, even if Neil's friend Luis didn't charge him for labor.