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Three useful choices and price points. All prices are guesstimates that will vary regionally:

1. Replace failed strut only- $700 installed/aligned. Still have three 1/2 gone stock struts that may be just fine.... or not.

2. Replace with M030 sport suspension. $1600 installed/aligned. Good ride quality, good performance, Porsche tuned to your car. Fresh on all 4 corners.

3. Replace with PSS9 struts. $3500 installed/corner balanced/aligned. Good (stock) ride quality at full soft setting. Fully tunable to your driving style. Adjustable rake and ride height.

A set of Bilstein HDs is an option but they are a generic strut that are not tuned to your car. Ride quality and handling may be better or worse but unknown. The Boxster is extremely sensitive to changes in balance, spring rates and dampening rates. They are also about the same cost as Porsche tuned M030 suspension.

FWIW I recently replaced my struts after 94k miles and they were pretty gone. I chose a set of PSS9s as my car is set up primarily for competition Time Trials. The car is now very good but I am not sure it is any better than a similar car with a fresh set of M030 struts/sways at 1/2 the cost. Choose the solution that is right for you.