Until just recently, I was assigned out in UT and had the great fortune to be near a lady (Destiny, who advertises on gunbroker) who is probably the leading civilian expert on grenade launchers. She is a master at welding the receivers to keep all threads intact and holes aligned. BTW, the Army goes to Destiny looking for M79 parts!
The receiver I got from her is a reweld, so that saved me quite a bit of money. $1,000 (friend price)
Barrel was a steal at $500 on gunbroker. This was complete with sights and ejector assembly. I just got plain lucky.
Misc parts probably about $300.
The hardest part to find was the trigger spring. Just a flat little piece of metal with an unusual bend to it. Finally found 3 of them. Used one, kept one as a spare and gave one to Destiny for her help.
I assembled it as far as I could then took the parts to her. In a few minutes, sitting at her table, she had it together. Noticed the hammer spring wasn't to spec, so she tweaked that part and put it back together. Functions perfectly now. Nice parkerizing, refinished wood stock and original sling. (I'm trying to find a photo.)
The key for me was patience in looking for parts at a decent price, with total time being maybe 3 years.