First - looking at chevy.com and building a Z06 with the Z07 package (carbon ceramic brakes) the price comes in at $7500.
I cannot imagine the Z/28 being more than this.
expanding on this spread sheet here is some data VIR lap times compared to price.
$112,000 ZR1 - 2.45.63
$88,585 Z06/Z07 - 2.53.50
$64,515 - CTS-V - 3.04.20
$57,885 Grand Sport - 2.58.80
$56,500 - ZL1 - 2.52.38
$36,035 - 1LE - 2.58.34
*VIR times are either lightning lap times or GM videos
**prices are from Chevy.com or Cadillac.com
Tried doing all these cars for Nurburgring but only came up with 4 times
ZR1 - 7.19.63
Z07 - 7.22.68
ZL1 - 7.41.21
CTSV - 7.59.32
SS was 8.19.xx but no 1LE time could be found
I still think Chevy can do this for high 40s low 50s but the price will probably be closer to 60.
Unless given direct goals from the top - it might be hard to place a price on this car.
I have also heard other car manufacturers say one thing and the release with something else - i won't believe the "more than zl1' until i see a release from GM then again the big Corvettes are going away probably until 2015/16 so the ZL1 and Z/28 might get a chance to flex their muscles. That is unless the Stingray is every bit the track car the Z/28 is and retails for 55k
Tough decisions by GM imo