Here's how to do it for free.
Two biggest limitations of Lab Manager, that caused us to use Xen with 802.1q for guest isolation, and iSCSI/ZFS cloning for template images:
1. Isolated groups of virtual guests can't span multiple hosts. Heck, we're doing that now with Xen and 802.1q.
2. You can't use VirtualCenter and Lab Manager to manage the same infrastructure - the one confuses the other. So you pay for ESX, getting DRS, VMware High Availability (HA) and VMware VMotion, but you can't use them.