Member, Sport: Off DSC: On (>50 posts)
Join Date: Nov 2013
Location: Denver CO
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It really depends what's wrong.
Are the struts shot/ leaking? They are no longer available new, so your looking at sending them to the company in Poland for a rebuild. Which I believe is around 1500 per pair and takes a few months. Mainly due to customs and shipping times.
Most other parts can still be purchased new and are shared with e34 touring models (except struts, these are M5 specific). I think the regulating valve and struts are the only NLA parts as of now.
As far as cost, hard to say, of course it depends on what needs replacing. If its just accumulators, budget $500 +/-. The whole system? 3-4k?
Keeping the SLS is worth it if you ask me. If you don't go this route I'll buy the old struts. I'd send them out to Poland for a rebuild... some day.