The costs to build a 'not so popular' engine to handle much more power than what it was originally designed to handle will be quite high. I'm interested in knowing the details of this build, ie: machine work, internal parts, etc.
Here is what I was quoted for my built VG30DETT built to handle 850+ WHP (~1k BHP)
Aftermarket parts for the 300ZX are more readily available and would be a bit cheaper because of the fact. Parts for the SVX would in most cases be more since they would be a less popular, made-to-order parts(assuming). Depending on the brand name parts and how much work is done, the $10K price tag, although steep, sounds accurate. Is it worth to pay that much to get 4-500HP? IMO, no, but some people are willing to pay that much. Im not too sure how robust the EG33 block is, but if people are making 1K HP race applications with them, they should do fine with just a piston and rod upgrade and some head work with cams.