I would say plea bargains at the federal most definitely occur under conditions of duress. It may not be physical duress (stress positions, water boarding, etc.), but the prospects you are presented with if you even attempt to prove your innocence would count as duress. The costs are prohibitive for all but the deepest pocketed individuals and even those with deep pockets are unlikely to have access to their own money since criminal cases give the government a lot of leeway in preventing you from using your money to defend yourself.