Not sure on cost as I haven't done mine yet as my forged build is on hold for a while, chuckie should know soon. We had it done to my mates 30 in his GTS4 and that thing cops an absolute beating, 8500rpm limiter launches and burnouts, still going strong.
Realistically, it should add much onto the cost of machining at all. Wouldn't take more than an hour to weld the snout up and if just fitting a collar the snout needs to be machined right down anyway.
On the topic of crank runout.... why would a splined setup ala JZ, not completely destroy itself?