Besides a free Hypervisor edition, VMware offers 2 different low priced Essentials Kits, which might might already provide the features you need. The Kits include vCenter Server as well as CPU licenses for up to 3 hosts, with a maximum of 2 processors each. Compared to the free edition, any paid edition removes the physical memory limitation of 32GB and also enables the vSphere API which is required by e.g. image based backup applications.
Please take a look at http://www.vmware.com/ap/products/datacenter-virtualization/vsphere/compare-kits.html for an overview and http://www.vmware.com/files/pdf/vsphere_pricing.pdf for further details.
André