Even if their home countries were to agree to foot the bill for airfare back to their home countries, how are you going to get them to agree to that? You'd need someone to negotiate details. That someone costs money.
Detainees will need to be processed by customs before being released. Custom's officers don't work for free. Do detainees even have their passports? Where are replacement passports going to come from, and who's going to pay for that?
Whatever your views on Guantanamo Bay, to think that it's not going to cost anything is simply naive. That's not to say that sufficiently motivated home countries couldn't pay for it, but it's going to cost someone somewhere.
Except if the home countries won't take them. Or if they will execute them if given the chance.
Nothing in politics (or life, even) is simple, so you might as well start by assuming it's hard and conclude it's simple after you've eliminated all the things that could make it hard, rather than the other way around.
