it really all depends on how close to the original you want the rifle to be.
plan to spend 7 to 800$ for a match quality barrel
250 to 400 for a 24" JLD or a 25" JOEKEN one
factory butt stocks for a PSG-1 run anywhere from 650 used to 920 new (920 was the quote i got from Fran at franshks he posts on sturm often)
complete trigger units are about 750$
forearms are IMPOSSIBLE to find right now but they go for about 150 i paid 200 for mine but i only got it because a fellow board member was kind enough to sell me his
correct cocking tubes can be purchased from JFK he posts here often and they come with an 21 style charging handle and all the internals for i think about 130$
the black-market-parts.com PSG-1 trunnion is actually a VERY quality piece, i was surprised, or you can go with an IGF trunnion extension.
the factory correct scope is very difficult to find,from what i hear the only way to really get one is to get one off of a demilled or parted out rifle.
i paid 925$ for mine and it came off of a demilled new zealand counter terrorist team rifle the gentleman who sold it to me had it in his safe for 10 years, and i had it shipped here from new zealand.
JFK also makes the factory correct scope mounts (the weld on tabs) as well as weaver tabs to mount a normal ring setup.
he also does the cuts in the bold carrier for the forward assist fora very fair price and turnaround time is awesome, RDTS makes a good copy of the factory forward assist assembly but that too needs to be welded on. he also makes the receiver reinforcement rails
it is alot of work and in CA you would have to make the 10 round mag permanent.
I'm doing mine myself and its not just the cost of parts, its tools also like a press to install the barrel, a park tank, tig welder, sand blaster and misc.
if you have any direct questions just PM me
here's the websites for the company's I've stated
www.RDTS.com
www.black-market-parts.com
www.invesmentgradefirearms.com (IGF)
JFK is the board member who makes alot of stuff also
good luck, you'll need it
