Many 401k plans have 0% fees and tiny fees on plan investments. I'm paying 0% fees on my 401k plan and absurdly low fees on the Vanguard Institutional Plus funds within it: .01-.04% per year depending on fund. (No that is not a typo.)

It all depends on the plan provider. The big ones like Vanguard, Fidelity, Schwab, etc., are all rock solid.