I make these boards with HASL (with lead). It's the cheapest option. ENIG makes the boards too expensive. And prototyping boards are not supposed to be expensive - they are only used to build a working prototype, then the proper boards should be ordered. Those should have goldfingers, like Carnivore2 cartridge that is assembled by 8bits4ever.
The HASL-made boards indeed have the slot contacts oxidized after some time, but that can be easily removed with a simple eraser. Just don't use the eraser with an abrasive component.
I don't agree with you about ENIG not being more expensive than HASL. For example PCBway.com charges 5$ for 10 boards with HASL, but when you select ENIG the price goes to 39$. And when the hard gold is selected, the price goes to a crazy 170$ for 10 boards. If you find some factory that has ENIG only twice as expensive as HASL, please let us all know.
ENIG makes the boards a lot more expensive, i also read it's not as easy to solder to. I wonder how much doing just the card-edge fingers with ENIG would cost, since that's the only part where ENIG matters.
By the way, i once bought a dirty two-decade old ISA soundcard from the fleamarket that was done entirely in HASL and it worked the first time without problems.
