Just priced these and bought mine a few weeks ago on fleabay (US).
For a 4,1 machine in stock configuration in perfect working order, and 8/9 out of 10 physical condition (few or no scuffs) and original box (important to me).
Single proc:
- Great price: $800
- Good price: $900
- Avg price: $1000
Add $300 to above for dual 2.26.
I paid $1000 for mine which was basically in new condition with original box, etc. If you have a couple of weeks to wait for the right deal, you can get them for less.
$600 - W3680
$500 - gtx680 mac edition (or equivalent)
$220 - 24gb 1333 ram
$200 - 256gb SSD
So your into it around $2500 for pretty much one the fastest mac pro configurations you can get. Going dual won't really get you a "faster" machine unless your applications can leverage the extra cores/threads, but if you do plan to add around $1600-$1800 more (assuming 2 x 6-core). Again, patience and research might lower that a little. If you want to start out with a newly refurbished machine from Apple, who's warranty you will immediately void, add $800 to the base price for a 2010 quad, and $1100 for a 2012 unit.
Now for the single proc machine, from a sheer performance perspective, you're probably paying oh around $1200 mac tax (and I'm being very generous). If your OK with that (I was this time), then there you go.