I've been thinking about this a bit...
What about DIYing the gold plating?
I googled and there are apparently a lot of "cheap" DIY gold plating kits around...
Do you or do you not want the nickel layer for low T-EMF applications?
Do you want a thin or thick gold layer?
Do you want pure gold or copper-gold?
Unfortunately I have no experience with DIY Gold plating, so I hope someone else can share some information.
Sor far my guess is, the DIY plating is only for a nice shiny look and will wear off as soon as you tighten the screws on.
The gold should be electro plated with no layer between copper and gold. Gold thickness of 6 µm seems to be adequate.
In my research I had problems to find pure copper spade lugs (I even build my own small batch).
TE/AMP and JST answered to my request, they do only have tinnend lugs.
Direct Gold plating is normally not offered when you ask for plating in the industry. They always suggest to use a diffusion barrier, but direct plating can be done on special order.
My best offer was around 50€ min. order value (plus tax and shipping cost), when I send them blank (untinned) lugs.