the mandrel bends to make a dump are usually $30 or so each, then the flanges are $40 or so each, so if youre looking at 2 bends and 2 flanges, thats a minimum of $150 in parts. then about 3 or 4 hours labour to fab it up, at $70 an hour, its $280 in labour.
worst case scenario, about $400 ISH for a full custom properly designed dump pipe.
I suggest looking at the ebay specials that come out of taiwan and see if they can be modded to fit. I scored a stainl;ess split dump and 3 inch to cat, made for a R33 but after about 30 min of cutting and re-welding it fits my R30 skyline like it was from factory.
Are you running a SR20det??? check out the Pulsar GTi-R...