It really depends on if you are the type of person who just has to have the latest and greatest, or are the type that seeks out a deal. You would undoubtedly pay top dollar for a MK III.

You might want to call Canon and ask about their Loyalty Program, where you can trade in your failing camera for a refurbished one. According to this post, http://www.overclock.net/t/1088761/canon-loyalty-program-faqs-originally-from-potn supposedly the 5D MK II is no longer available, but it never hurts to ask...you might be able to get a 5D MKII for a very low cost.