@deanwinchester He's spent somewhere in the range of $20,000 to $30,000 on her in the last year. If you remember chapter 1, his parents gave him 1 million yen in exchange for no allowance. That's about $10,000. He quickly ran out of that money (each date was about $500, and he was going on a date with her every week. His first date, it showed that he paid her 40,000 yen, or about $400. With a date every week (every wednesday I think) for a year, that's $20,000. He's also spent money on dates with those other girls, plus all the other times he's spent money for her (like the movie stuff, extra dates, etc).
The only way this story has any redeeming quality is if it turns out Mizuhara has been dating someone for a while or starts dating someone else other than Kazuya. I won't accept any other outcome other than failure.