10'000 Hours/ Getty Images Ensuring your 401(k) is delivering the best return possible is essential for your retirement. Key Takeaways A new study found that more than half of 401(k) plans from 2009 to 2013 offered consumers at least one investment fund option that shared revenue with the plan's administrator. That means higher hidden costs, which can add up to thousands in lost value by the time you retire. Do you really know how your 401(k) plan is invested? If not, you might be putting your money ...