LS7 is ~ $7,000.00 premium over LS3.
Then there's the supporting drivetrain, HD gearbox and diff. $3,000.00?
It has to stop as well as it goes, add ZL1 brakes $2,000.00?
Lighter wheels & fat tires $2,000.00? (easily)
Struts/springs/sways/bushings $2,000.00 (conservative)
GFX?
..... put the 427 in the LS and call it a COPO... oh wait...
Except for the LS7 price, the above is just a best guess, but a LS7 Z/28 would be close to ZL1 price.
Don't get me wrong, I think it's a GREAT idea, but GM isn't gonna build it to give it away.