the extra cost is from the stronger degree of evidence and exhaustive appeals process required before the inmate is killed, right?

Some of it is from more expensive incarceration, but you're right. This has one detailed breakdown:

• Extra defense costs for capital cases in trial phase $13,180,385
• Extra payments to jurors $224,640
• Capital post-conviction costs $7,473,556
• Resentencing hearings $594,216
• Prison system $169,617

However, we're assuming that with cryonics as an option the entire process would stay the same. That needn't be the case.