What's your milage? Befreo jumping into injectors I would at least do the fuel filter ($20) and check valve ($11) first. A rough idle could be lots of things. It'll cost $250-300 for new injectors and associated parts. I would run as many tests as possible to be sure that's your problem. There are lots of good diagnostic tips in the archives, its worth the read.
1988 420SEL 170K