Actually, even if the trial is expensive for capitol cases, it's a set expense.
Would not housing them for the next 40 years not be more expensive then giving them the black needle? Unless you just don't go for the death penalty at all I guess.
In that case, making them chain gang and make big rocks into gravel without the expensive capitol case would certainly suffice.
See, you made me not want to have them shot at a shooting range for Army round testing, nor chased by lions at the Super Bowl halftime.