I'll be critical.
$250k per annum is a small amount. You can make it through a variety of means - being an underwear model, a children's book writer, a makeup artist, an eBay store owner, a web forum manager, a daytrading coach, a wastewater recycling startup founder, a network traffic consultant, a restaurant owner. I know a multi-millionaire for each of the above jobs, half of whom were mathematics majors. You can do any of the above with a mathematical science degree with a computer science background. In fact, it's an edge that you have a mathematical background in a field that lacks quantitative expertise, because there's more low-lying fruit. So finance is, in some ways, your worst bet if your goal is to make a lot of money. (Not to mention that you probably have a long way to go if you associate economics with finance.)
I know a firm that pays $170k for a B.Sc in math fresh out of college, but that means you have to compete with hundreds of applicants for a couple of positions, and you need to compete with ten thousand people with the same background as you while you try to get promoted to that $250k per year. (And if your degree is in 'Mathematical Science' and not 'Mathematics', chances are that your resume will get trashed before it gets read.)
Many people claim to be motivated by money when they really aren't willing to work as hard for it as they think they will. They're interested in shortcuts, how to avoid risk, how to make it with the least effort. You asked in the title, "How to earn a massive income" but you're really asking for "What jobs earn a massive income?" This is the sort of question they ask. It's probably the laziest mindset if one is hoping that having a particular type of job entitles one to a particular income.
Another piece of advice: It's perfectly noble to have a dream of making a lot of money, and some of the people I respect the most tell me upfront that their lifelong dream is to make a lot of money. But if that's the case, you should aim for something substantial, like $100 billion.