Make sure he is experienced with the subject matter of your particular case. An attorney who has handled 100 federal drug trials may not be as competent handling a complex federal securities fraud case and visa versa. Remember that you usually get what you pay for. Excellent attorneys with established reputations will charge more than others, and the range of fees will increase with the seriousness of your case and anticipated length of trial. Remember, a retainer is an up front fee and doesn't necessarily embody the entire amount you will end up paying, especially if your case goes to trial.
Most attorneys charge by the hour.
Be sure to inquire about this. Ninety eight per cent of all cases that go to trial are won by the government. Nonetheless, the best lawyers obtain the best plea bargains because they put more research and effort into trial preparation. That's why the best criminal defense lawyers put equal amounts of work into cases they end up pleading a defendant guilty to as they do in cases that they've prepared to take to trial. So if your prospective attorney requests a small retainer, you can rest assured that he intends to plead you out quickly.