Massive VMware Bug Shuts Systems Down
You don't understand ESX licensing. The $5000 ESX license is for a VMWare host, not for a virtual machine inside. So you pay a total of $13000 for an ESX server that can host as many virtual servers as the hardware will handle.
Also, blades are generally a bad idea as ESX hosts because you're putting all of your eggs in the one blade center basket. One of the main reasons to go with ESX is to get the ability to VMotion virtual servers to different ESX hosts in the cluster if one ESX server dies. You lose some fault tolerance if multiple ESX servers are in the same blade center.