At first I was pretty mad at the ending. First, why does it matter that they're rich? The whole point of everything that they did and how they fell in love was that money wasn't a part of it. (Also is a theme of the manga, that life is priceless)
Second, the girl just throws her life away then? because she would have still only had three days with the MC. It was only her first love. She still had her whole life ahead of her. it makes the MC's sacrifice pointless.
However
Looking at this as a tragedy, like Romeo and Juliet, it makes more sense. This is a story about some young / naive individuals. It's sad because we know that there's more to life out there. We ourselves know even when the MC sells his life for barely any money (its like 3K USD) that he's making a dumb decision. Those of us that are older and have some life experience know that these sacrifices are dumb. Just like taking poison just because you think you're first love is the bomb.
It makes you want to scream No it gets better, I swear!
I hope that perspective helps some of you. I was a bit lost for a day or two until I had that realization.