Well, in the least I don't think halo cars contribute to a direct/traceable increase in sales of other products or to the bottom line of the automaker. People are smart enough to know, if but only based on price, that the GT-R and Altima are nothing alike. I'd also guess 80-90% of Nissan customers have never hard of the GT-R or of the storied history of the Skyline in general.
Halo cars are about marketing (in getting the automaker name out there, which the GT-R has been hugely successful at), it gives the company's hotshot engineers something to stick around for and it serves as an internal morale boost. The Ford GT (2nd gen), Dodge Viper, Lexus LF-A and Honda NSX (original) have been other very successful halo cars.