Your only comparing up-front initial layout costs, and the difference in product isn't as simple as Jockey vs Fruit of Loom.
Lets do look at the product comparison first - especially what I call the "bullet list problem".
If your going to compare to products like this one of the first things your going to do is just compare feature sets. Both have probably 90% overlap in the "Feature list" but that is kind of deceptive... not all features are equally valuable, and quality of implementation matters too. A feature you're afraid to use in production, because its proven itself to be "unsafe" doesn't really count. How easy is it to use the feature? Does it mesh with your existing infrastructure? Does it have an API? What limits / restrictions are built in? A fixed bit crank drill is cheaper than a multi-bit power drill, but if it doesn't suit your needs, its a waste of money.
Support - how much does the level of support you need cost?
What do you get, how good are they, what are the SLAs?
What will it take to support the env in terms of people, and training.
For example compare the linux KVM project and VMware - If i'm doing university research grid computing - low budget, lots of smart students / sunk cost research assistants, and no real availability SLA's then KVM is a great way to turn 50 servers into a very flexible computer cluster. Throw people at it and let them go all artisanal on it until it mostly works most the time.
But for the virtualization platform for your production env. for you core revenue stream? I want something that I can easily get my people trained it, easy to hire experienced talent / consultants, and so standardized that we can write up pretty comprehensive play books for over night support.
Ecosystem - VMware is the industry leader. That means that if a 3rd party is going to pick who to support, or who to support best, its going to be VMware.
So all rolled up, it means that its not a apples to apples comparison, you have to take TCO into account (including salaries, % of employees time spent there rather than elsewhere, "loses" due to outages / slow recovery times, secondary costs etc), and risk management as well.
I've dealt with Virtualbox, Xen, XenServer, KVM, and VMware and my unreserved take is that if you can afford it - use VMware. The barrier to entry is high for licensing costs, but if you can afford it, the ROI is better than anything else. If you can't afford it, get your foot in the door with something else, then once you prove the value of virtualization, use the next time your first platform fails hard as the leverage to get VMware ;)