Presumably, AMC can handle paying for twice the hours they originally wanted, provided it's spread out over 24-months. The network have been playing a risky game this year because of financial pressures. The future of Mad Men was in doubt for a few months when they played hardball with Matthew Weiner over costs, and it's believed they fired Frank Darabont from The Walking Dead over constant arguments about plans to cut the zombie drama's budget.
The problem AMC have is that they have three high-profile shows, but not necessarily the money to pay for them—as only The Walking Dead is enough of a mainstream hit to justify the running costs. Mad Men and Breaking Bad may be critical darlings that win awards and help promote the AMC brand, but they've never been ratings juggernauts. And the bottom line is AMC's shows are paid for by subscription fees, so need to draw eyeballs.
AMC actually produce The Walking Dead themselves, so can enforce whatever changes they like to that show—unlike Mad Men and Breaking Bad, which are made by independent companies Lionsgate and Sony, respectively.
The only hurdle for Breaking Bad now is signing showrunner Vince Gilligan for the final 16 episodes. His contract expires at the end of the current fourth season, so will he happily just extend his existing deal, or will he use his position to negotiate more money for himself? That remains to be seen, but I expect Breaking Bad to continue for two more eight-episode seasons, finishing in 2013, with Gilligan at the helm.