To my mind Bruno pulling out has less to do with money than the fact that they have reviewed the evidence and they now know he is guilty of both kidnapping and murder and will be found to be so. They probably hate him even more than most after what they have learned and simply cannot/will not defend evil of this horrendous level.
At the most basic level, the attorney's duties are to ensure that the rights of his client are not violated. On a realistic level, they are out to make money and this case represents a lot of money and sudden prestige for the younger Brunos. Who in their right mind would hire a 20 something to defend them against kidnapping charges in federal court? Tom has been an attorney in the community for a long time. They don't care whether BC is guilty or innocent. Bruno himself suspected that he may have been selected through an Internet search. Bruno doesn't advertise but this kind of exposure is better than any advertising.
Lawyers need to choose their clients carefully. They can, and do, ask for retainers ahead of taking cases. Because they chose to represent a client who could not pay may not be a good enough reason for a judge to grant permission to withdraw from the case. When they file a notice of appearance with the court they are asking the court for permission to represent the accused. The court can, and does on rare occasions, decline their request (I vaguely recall the judge in OJ Simpson's case denying permission because he felt OJ had enough representation already). This is why they must request permission to withdraw.
Janie Miller is not the public defender for this court. This is Federal court. The Brunos may ask for permission to be withdrawn as counsel for financial reasons but it is unlikely that the judge would grant the request on those grounds alone. In this court, the judge would be more likely to essentially assign the Bruno's as the public defender. I think the Central District of Illinois only has 3 public defenders. That would leave the Central District without public defenders until this thing goes to trial.
The Brunos would submit their bills to the court for payment. This kind of arrangement is not uncommon in capital cases and high profile cases. I suspect that the Brunos were well aware of what they were getting themselves into when they took on a case with a client who does not appear to have the means to hire them.