The conversion to do it properly will cost around 9 to 10 grand unless you shop for bargains. You will need a fair bit of customizing sumps and exhaust stuff like that costs a fair bit plus the engine with a 3 grand price tag along with ecu and loom. You also need to decide if you want to keep it stock or upgrade things before hand.
Shopping list
Rb26 2 to 3 grand for a decent one
Loom and ecu 400
Sump conversion 300+
Front pipes 300 to 500
Front mount with piping 500 to ?
New gaskets and other good to buy bits 500+
new heater hoses 300+
safety money 1000
Labour 4000+