Where is the most economical and best place to get all my wiring harnesses for my Chevelle? I would like to order all at the same time.

I dont know about economical but I just installed a new harness in my 70. Got it from M&H on recommendations from people on this site. I had them upgrade the fuse box to blade style fuses and added a couple extra circuits. It was plug and play and when I was done, everything worked!! Excellent quality and if needed, tech support. Wasnt cheap but the harness quality was A-1 and was an EXACT copy of the original except the changes I made.
American Autowire is also highly recommended on this site. I wouldnt want to be cheap when it comes to a wire harness!

That's a weird deal. I've done some research on this as I also needed ALL the harnesses for my '69. Looks like there is some kind of cartel that sets pricing.

Consider the case of a dash wiring harness for a '69 with gauges and no AC:
It all started for me when one of the eBay vendors was selling one for $419.
M&H Electrical Fabricators makes the harness and sells them for $419.
Ground Up sells them for the same price with no discount available.
The Parts Place also sells them for the same price.
National Parts Depot is $458
Original Parts Group is List of $459.95 but promotional price of $413.96.
Muscle Factory sells it for $359.95 but I'm wondering who the supplier is. Probably worth a call to them but it's hard to imagine there can be such a wide difference in price.
Good luck.