I'm in the middle of doing this research myself. There's a Facebook Group called LowBuck LS which has some good info in it. Recently a guy posted he does standalone harnesses. A drive by cable, 4L60e, Gen 3 harness was quoted at $270. Given how many hours I would likely spend dieting and extending OEM harness to fit where I wanted it, that sounds like money well spent to me.
I would recommend asking some questions like what kind of connectors he uses and what features are enabled (how many O2s, electric fans, a/c, etc). I have seen some really bad low-buck harnesses like that and they are a nightmare to deal with.
