does anyone know what type of cost im looking at to dump an a15 or ca18 or even an sr20 into an
81 datsun 1200 ute?

i just need a rough idea

As you are asking costs I'll relay what it'd cost to have someone else do it for you.

Prices in AUS$.
A15 -> < $1000
SR20 -> $6000
CA18 -> $6000
CA18DET -> $7000 - $8000.

The A15 requires very few mods. Thus is much cheaper.
Remember, these days expect $50/hr for any mechanical work. A typical conversion can take a week of solid work, even if it's been done before.
So that's at least $2000-$3000 labour.

The engine/gearbox costs range from $900->$2500 depending on combinations. And the electrics/computer can cost some more. The EFI new engines are nice though. But you've gotta love 1200s to
go down this path, because you could buy a newer car with this technology 'already installed'. So it's not justifiable in a money saving sense. All in non-turbo form are road-registerable,
but the SR20DET is not. People are registering them without a turbo and putting them on after the engineer inspection. They are in for ~$2000 in fines after the police pull them over and
run the numbers. It immediately voids the registration. Thus driving an unregistered/uninsured vehicle etc etc.
Police do know what a turbo looks like.

Take it easy,
Chris.