Dual XEON E5520 @ 2.27GHz (core i7s, 16 virtual cores)
Quadro FX 5000
When I rendered GPU only, the render time for 100 iterations went up significantly. In my case, the CPUs play a significant role. PiXeL_MoNKeY, in your case, I can see that with 24 badass GPUs, the CPU is probably just getting in the way.
We also see the logarithmic scale of cost vs speed. I have one Quadro 5000 and my render time is only twice that of yours... or did you render at higher resolution?
Regardless, I'm envious of your GPU farm. Is that a shared resource in your studio? Tell us more about your setup.
100 iterations: 31s
10 min render:
The quality and render time of my rather expensive rig is almost identical to the OPs results at spec - CPU : i7 860 (3.4oc), GPU : GTX460. His was 33s. I copied his 10min image and compared it to mine in PS and they are nearly identical.