washer mod if you just want noise, cheapest solution out there
if you want something like the top speed axle back, look up G37 axle back on ebay and you basically get the same thing but for much cheaper(175 was the cheapest i have seen an axle back go for)
EDIT
they went up abit but here is the cheapest one i could find in the 30secs i spent looking for it
http://www.ebay.com/itm/09-Infiniti-...fd9746&vxp=mtr
iirc the topspeed axle back is a tanabe axle back replica, so buying any tanabe replicas should give you the same sound as the top speed axle back