Depends where and what you buy, and who you get to do the work.
20k could get you a seriously fast 32 GT-R even if you started from stock, but as you already have a fair few sweet mods it would go a LONG way.
Think about it, start with a stocker;
sell the 26 bottom end
sell the standard cooler and piping
sell the standard injectors and fuel rail
sell the standard fuel pump
sell the airflow meters
sell the standard turbos
sell the standard dumps and exhaust
sell the standard ecu.
I would think there would be a few grand in all that.
Now, doing all the work yourself you could have a built 30 bottom end under your 26 head with 4wd adapter for 7k or even less I'm sure.
now go buy 1000cc injectors, fuel rail, fuel pump(s), intercooler and pipes, -5 turbos, nice big exhaust system , ECU,boost controller. I've probably left things out but i think that would come in under 20k and you would have a 400awkw GT-R.
Mind you this would be ghetto spec modification.