Recent Infrastructure-as-a-Service offers, such as Amazon's EC2 cloud, provide virtualized on-demand computing resources on a pay-per-use model. From the user point of view, the cloud provides an inexhaustible supply of resources, which can be dynamically claimed and released. This drastically changes the problem of resource provisioning and job scheduling. This article presents how billing models can be exploited by provisioning strategies to find a trade-off between fast/expensive computations and slow/cheap ones for indepedent sequential jobs. We study a dozen strategies based on classic heuristics for online scheduling and bin-packing problems, with the double objective of minimizing the wait time (and hence the completion time) of jobs and the monetary cost of the rented resources. We simulate these strategies on real grid workloads in two cases. First, we use the workloads as a whole, which is representative of a large community of users sharing some common resources. Second, we use the workloads extracted for each individual user. These lighter workloads correspond to users submitting work independently from others and paying for their own resources. Our findings show that on large workloads, a little budget increase allows to achieve optimal wait time, while trade-off heuristics may be largely beneficial for individual users with lighter workloads.