I've seen them as low as $2300. Currently $2600 on e-bay.

KW Suspension Variant 3 Three coilover Kit 01 06 BMW x5 E53 35220053 | eBay

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'03 X5 4.4 Sport, last of the M62s (8-03 build date)
I believe in deadication to craftmanship in a world of mediocrity!