The current 1G components are already 4x-8x that cost in volume. Much like 100M never seemed to quite go away for decades (and still hasn't in certain areas), this doesn't need to hit the bottom barrel to still hit cheap motherboards. Especially for people buying by components, many "entry level" A620 motherboards launched 2 years ago already had 2.5G NICs.

It may be a bit longer before random PCs at Walmart have 10G more often than not but it won't be long at all for "mid range" motherboards you're talking about.