Yup, I am also an M5 owner. I can't really recall how much I paid for the Stage 3 suspension since I had strut braces installed at the same time. I think it around 4~5K labour, taxes all included (stg3 suspension alone). I had it done like 1.5 year ago, and the exchange rate is better now. I think you should be able to get it cheaper than I did.
My guess is the H&R coilover would cost around 2K US.