A financial planner says too many people make the same mistake when weighing the cost of going back to school

For the rest of us, it's not so simple.

Brandon Moss, CFP, Director of Client Experiences at United Capital, says when you're weighing the cost of going back to grad school after a few years of work, don't think about what you'll be earning once you graduate. Think about what you'll be earning for the next five to ten years.

"Be sure that you have a very strict plan," he said. "I'm doing this to maximize not what I make when I graduate, but I'm doing this at 35 so at 45 I'm hyper-positioned to be massively successful and take advantage of the next 10 years. The period of ages 35 to 50, for most people, are your peak earning years, so whatever I can do in my 30s around school, I want that to max those peak earning years. I don't want to be as concerned with what I can get at 36, but that what I'm doing during school is going to maximize these peak earning years for me."

Data compiled by PayScale suggests that most people will never earn more than they do in their 40s, their peak earning years. College-educated men's earnings peak at an average age of 48, while women's earnings peak at 39. (PayScale attributes the disparity to job choice and family responsibilities.) By their 40s, most people have completed their education, secured some promotions and raises, and earned accordant paychecks.

Moss said a lot of people do "money grabs" in their 30s, but don't think about the future, about the point at which their income peaks. "With student loans and what it's costing, I want to be sure that I've got a very good plan around why I'm going," he said. "I want to be sure that I'm picking the right school for what I want to do, not just that I'm going to the local best university because it's what I can swing. If that's not really going to enhance your human capital for exactly what you want to do, you might just be blowing $100,000."

His cost estimate sounds outlandish but isn't off-base. The bill for school can be steep, which is why it's so important that it's offset by future earnings. At the Wharton School, for instance, tuition and fees cost $73,634 annually - but graduates earn an average salary of about $155,000. At Harvard Business School, annual tuition and fees are $75,353 - and the average starting salary for graduates is about $154,000.

Your 30s, Moss told Business Insider, are your time to "maximize your human capital. Whether you're going to school or changing jobs, this is the right time to do things for the right reasons."