Calculating risk vs cost for server storage

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http://serverfault.com – This is a request for 2 different types of information: Is there a formula that can/should be followed to calculate risk vs cost of server storage? What factors should one take into consideration when deciding on what kind of server storage to purchase? By server storage, I mean choosing between getting SSDs (consumer or enterprise grade), consumer spindles, raptors, sans, etc. (HowTos)