Ive seen them for less then 50. I just see it as convenient to do everything at once while youre under there. I mean do you want to take all the covers off 6 separate times and have 6 different misfires. If one is going the others aren't far behind. Just wise words of wisdom esp with 112k, Set it and forget it one less thing to worry about.
here is from ecs tuning, dbl check your part numbers this was just a quick search.,
http://www.ecstuning.com/BMW-E90-330....0L/ES1304983/