The users of Cloud Service Providers (CSP) often motivate their choice of providers based on criteria such as the offered service level agreements (SLA) and costs, and also recently based on security aspects (i.e., due to regulatory compliance). Unfortunately, it is quite uncommon for a CSP to specify the security levels associated with their services, hence impeding users from making security relevant informed decisions. Consequently, while the many economic and technological advantages of Cloud computing are apparent, the migration of key sector applications has been limited, in part, due to the lack of security assurance on the CSP. In order to achieve this assurance and create trustworthy Cloud ecosystems, it is desirable to develop metrics and techniques to compare, aggregate, negotiate and predict the trade-offs (features, problems and the economics) of security. This paper contributes with a quantitative security assessment case study using the CSP information found on the Cloud Security Alliance's Security, Trust & Assurance Registry (CSA STAR). Our security assessment rests on the notion of Cloud Security Level Agreements - SecLA - and, a novel set of security metrics used to quantitatively compare SecLAs.