For many workers, saving for retirement is a priority. The rule of thumb is to start saving early so the money has a long period of time to grow.
According to the Investment Company Institute, 401(k) plans have become the most common defined contribution plan and hold a whopping $4.7 trillion in assets.
But those plans don't come free, and finding out how much they cost can be a difficult task.
The spotlight on 401(k) fees comes after a recent spate of lawsuits against companies like Boeing, Lockheed Martin and Edison International, where employees claimed excessive fees and poor management of retirement plans.
That said, the threat of litigation may be driving down costs. According to BrightScope, a website that rates 401(k) plans, the average plan participant paid a total cost of 0.89 percent of assets in 2013, down from 1.02 percent in 2009 on a plan-weighted basis.
So how can you find out how much your plan is costing?
The widely used 401(k) plans "are the dirty little secret of financial services," Stacy Francis of wealth management firm Francis Financial told CNBC's "On the Money" in an interview.
"They're quite complicated and truly understanding the plan administrative fees is difficult because often they're buried," she explained. "And there's millions of dollars that are being made by companies without the average plan participate knowing."
Francis added that "the first thing you can do is get your plan benefit book."