1280 x 1024 @ 64k colors?

Between US$300 and $650 for the exact same board depending on who you buy it from and what the context of the sale was. The cheapest route to this resolution with decent performance is a Via Eden or C3-based board with the Apollo-series companion chipset (containing an S3 CRTC/LCDC). BYO RAM and boot media.

There are some boards with [mostly] 486-class CPUs and C&T (Asiliant)

69030 controllers that will support your target resolution, but this is hoary technology. A 486 doesn't have what it takes to blit around this much display memory.

Places that advertise retail pricing on this sort of hardware are usually resellers putting very large margins on the products - they buy in relatively small quantity, and add their own margin on top of that already high price. In order to get real prices, you need to go through the tedium of calling the suppliers, I'm afraid.

Some vendors - like Advantech - offer online shopping for _some_ of their products but not all. IEI (BCM/eValuetech) is cheaper than Advantech for more or less identical products (even down to the same mechanical layout) but they are harder to deal with.

It's hard to work out how many different manufacturers of this stuff there really are. When they all use the same chips and the same BIOS code, and all seem to suffer the same bugs which all get fixed [or not] at the same time, it seems suspicious that there is just one factory in China somewhere turning out all this stuff.

CLARIFICATION

3.5" isn't THE dimension spec, it is the largest form factor I would use.

3.5" or smaller.

2.5" biscuits are more expensive. SOMs are expensive to get started with but can work out cheaper in volume. I'm not aware of one that meets your res spec though.