The drugs themselves are expensive, but a drop in the bucket it seems to the legal costs and death row expenses. Arizona two years ago bought a large batch of chems at $1.5 million for a supply. The chemicals go for $1,500 a gram and they are required to use 5 grams per execution. Not cheap, but certainly not the budget killer.
The problem is the appeals process and accompanying forensic evidence testing it seems. Some states will carry out appeals for 20 years or more, and for these kind of trials they stipulate that the litigators have to be seasoned which means expensive. They also require additional lawyers over traditional trials, adding to the costs with each appeal. Meanwhile these death row inmates are in separate blocks in individual cells with added security, all additional costs. One man that had his sentence commuted to life in prison said moving from death row to gen pop was worse than death. Not only are the accommodations worse, but you stand the chance of being raped, stabbed, or worse.
All this says to me is the system is inefficient and bogged down by bureaucratic bullshit. If the evidence is damning I say grant one possible appeal and that's it. If you've got conclusive video & DNA there shouldn't be a need to tie up the system for years.