A lot of this all depends on how much you are willing and capable of doing yourself vs. highering someone. Installing speakers, while easy, is still time consuming. From an installer viewpoint, if you own speakers, the installer still has the headache of ensuring that the speakers will fit in the space where the wires go to, they then need to make sure that every single wire is properly labelled on both ends so that it can be properly found, then the hole must be cut with care to ensure that no ceiling joists are inadvertently hit. It must then be connected to the amplifier, preferably with some sort of clean connection (banana plugs, etc.) and then tested. It's at least an hour of work per speaker pair to get installed. No keypads, no programming yet. This is also almost the baseline requirement no matter what system you intend to use. So, individual zones, or one big zone, this time and effort will be the same.
Obviously, a big difference between 6 zones and 12 zones when we are breaking it down like this.
Now, you look at a single amplifier setup, with in-room volume controls or a volume control/speaker selector, and a couple of used amps, and you can CERTAINLY do this system for not a lot of cash. Save more by doing it on your own even. It may not be pretty, but it can be done.
I always go with what I'm familiar with, which is Crestron, and tends to be more expensive, but I pair it with used/b-stock gear and it isn't outrageous as it could be. But, I do like the Sonos as a workable idea and you keep your in-ceiling speakers.
Put a bunch of these where the wires need to connect...
http://www.sonos.com/products/zoneplayers/zp120/default.aspx?rdr=true&LangType=1033
Then put your in-ceiling speakers in.
Of course, for 12 zones, it's $6,000 and you still need to get individual controllers for everyone and it isn't installed, and there are no speakers included.
The flip side would be to get everything you can used and do as much work as you can yourself.
If I were looking at something like this:
12 pairs of in-wall or in-ceiling speakers - $80/pr - $960
2 - 6 zone 30wpc amps - b-stock - $600/ea. - $1,200
12 - 12 button keypads (each room) - $100 - $1,200
2 - 8 zone preamps - $500 - $1,000
1 - Controller w/power supply - $500
Installation & programming - $3,000
So, about $8,000.
The speakers aren't terribly expensive (or overly impressive), but everything else would be rock solid and give you a lot of capability.
This is a typical multi-zone installation without wiring and there would be some extra hardware required and it would be a VERY good system. But, using used gear. So, there is several thousand in cost savings which can be gained by going with used gear.