Sure... you're going to want to spend less than $3000... but you're going to use the original timing belt? original water pump? etc... You have to consider the parts involved and labor, intercooler, piping, materials, driveshaft? shipping... etc... you're gonna keep stock rally height on the Z with stock tires, stock brakes, stock everything and safely operate a 240Z with a RB20 or RB25?
Well yes maybe you can get it all installed and running for under $3000 (if you manage your funds very closely)... but would $3000 be enough to safely operate this vehicle and will this old RB20 thats about 20 years old run reliably? maybe... Would I budget $3000 dollars for a RB20 swap to be cool? No.
EDIT: More power to you if you can do it all for under $3000 and it makes you happy... it's not my car and I'm not driving it. Good luck