I reckon 50k minimum if you buy brandnew parts for an absolutely stock rb26.
You can do it a lot cheaper if:
1. There's already a lot of tuning parts in the car when you bought it.
2. Buy used but still in good working condition parts.
3. Sensible mods eg. Garrett instead of HKS, stock IC instead of big, stock radiator, induction piping etc....
Remember I've got for sale a basically complete rb26 with forged HKS pistons for a lot cheaper than 10k, a complete induction and turbo kit and downpipe for a lot cheaper than 10k, and also 700cc injectors. The complete lot for much less than 20k hehehehehe sorry for the promo
BTW, my turbos has been checked by turbo specialist and are fine to use
With everything stock you won't ever get near 600hp laa
The engine just doesn't flow enough air - you will need bigger turbos.
Plus you need more fuel - stock injectors are only 444cc.