I can't say how much as it depends on who you get to do it ... your best bet is to talk to someone who has performed the conversion close to where you are.
That said, I'll estimate $300-400 as a rough ballpark figure to cut & wield the manifold itself. At least, that is how much it cost a mate to do it up here in QLD.
How much the parts cost alone is something else - most importers do not sell the parts separately (up here in Qld anyway) so unless someone on these forums or a wrecker is selling one in parts ...
Someone who doesn't need it may give it away for free whereas someone who knows its worth may sell it for $150.
Just like when I picked up a 4AG bluetop engine for free this week when the previous owner was willing to scrap it for nothing.