1. A method comprising: receiving a representative workload of a computing system, said representative workload comprising at least one composite transaction; determining a resource cost for the at least one composite transaction, said resource cost reflecting an amount of utilization of at least one resource of the computing system in serving the at least one composite transaction, the amount of utilization determined by U CPU , n ′ = C 0 , n + ∑ i ⁢ N i · C i , n T ; wherein T is a length of a monitoring window during which transactions served by the computing system are monitored; U′ CPU,n is an approximation of the average CPU utilization of the computing system during the monitoring window at the n-th tier of an application hosted by the computing system, wherein n comprises at least one tier; N i is a number of composite transactions of an i-th type of composite transactions, where 1≦i≦M; and C i,n denotes an approximated average service time of said composite transactions of the i-th type at the n-th tier.