Mine was going to be 6500. But since you have an LBZ it will be a little more since you dont have a set of LB7 pistons laying around. I know guy said 2300 for a new set of Lb7 pistons cut and coated not sure what curtises price is but for that money you could save some coin for some MAHLEs. Either way the piston cost is going to be higher for you. You will also need to add 650 to that since you dont have head studs.
My current stock build has cost my $400 for headgaskets, seals and some bolts and lube and litle things.
20K is very realistic and actually a little low for a good ground up build.
My build was just to get me by. Nothing in my build did anything to help with power, just allow some more.
To build a motor and give it the air and fuel it needs you will need a lot of money.
But if this build was just 20K into the actual motor itself i can see it being very realistic. Heres an expensive build price for you.
Socal 7.1L Stroker $11,800
Optional piston coating $400
Socal Stage II heads $4000
Optional Titanium Retainers $200
Socal Custom Cam $912
Socal Billet Injector clamps and studs (LB7 only) $550
ARP Custom Age 625 Head Studs $1,900
Set of C headgaskets $220
Chrome moly push rods $330
Billet Mains $440
Main Studs $280
Id expect this build to be at least $25,000 with machining costs, other little pieces and if you had somebody else assemble it.
Even if you didnt go with the stroker kit it would still only be a couple thousand cheaper.
Now you still have to add the fuel and air to it. Air can be done on the cheap side by doing budget twins for about $1,000. But ive seen some setups knocking on $10,000.
Fuel is expensive. Sending you injectors out to be Extrude honed costs between $900-$1,200. Twin pumps are nearly $3,000 now for a kit.
And then you still have to worry about blowing a hole in the intercooler which is at least $1,200 to upgrade.
Not to mention now that your pushing that much power, youll need to be refreshing that suncoast every year if you plan on using that power for competition.
It can get very expenisive. I bench raced a build that would cost $45,000 using off the shelf parts. Thats everything involving the motor, fuel and air systems.
The possibilties are endless. Its the wallets that slow us down.