How cheap is cheap. $330 USD?
2011 v3 ES Xeons can be expensive. Even if they are still less then the retail price.
For me personally a Xeon ES would be fantastic. But please do note, that not all ES CPU's work with certain motherboards.
There is a thread over at STH that has a community of people talking about building computers with ES CPU's and buying ES CPU's. https://forums.servethehome.com/index.php?threads/es-xeon-discussion.5031/
It might be wise to ask them what ES cpu's work on your board. They seem to be a very helpful community. Also, someone who has the same board as you posted on that thread. https://forums.servethehome.com/index.php?threads/es-xeon-discussion.5031/page-48 It looks like the 2666 v3 works with the board. On ebay it looks like you can get it from two vendors, one from Japan for $369 USD and one from Texas for $550 USD. I would rather go for the Japan one.
Also ES sample CPU's may not have all of the features or spec of the final product. For example the 2666 v3 is a 2.9 Ghz cpu but can turbo up to 3.5 across all cores. The ES sample that you may get may not be able to turbo that high.
Looking thru that thread I linked above, early ES samples of the 2666 v3 do not have HTT support.
Excerpt from post #949
The early samples of 2666 v3 do not support HTT (W/O HTT there will be a 10-15% performance hit, depending if the application can utilize HT. Seems that the HTT is greyed out, so it is permanently disabled from the chip.
I believe @Cavemanthe0ne knows more about ES cpu's more then I do. He talked about them briefly before somewhere else. He might be able to steer you in a better direction.