There are 2 sets of fees in a 401-K.
First there is an account fee. They are required to disclose this to you, but it can be hard to find. My 401-K has a 6.5 basis point account fee, which is 0.065% of the balance. This is a pretty awesome low fee. Then above that each fund you invest in has an expense ratio, often abbreviated ER. This can vary widely based on what is available, from 0.1% for a good index fund to over 1.5% for very expensive managed funds, and everywhere in between. My 401-K has a few index funds with fees below 0.20%, which I'm grateful for.
If you are in a position to roll your account over, look to Vanguard or Fidelity Spartan funds. Either of these has good low cost options. For almost all circumstances, you have to keep your 401-k at your employer until you separate from employment.