Multigig switches are expensive because they're new. The products have NRE expense to pay off and so do the chips inside them. There's not yet the volume to make them really cheap. They will get cheaper.
The question is will they get cheaper faster than 10G? Perhaps. Despite being 14 years old 10G is just starting to get put into reasonably priced prosumer switches so it doesn't have that much of a head start practically.
I have been thinking for a long time if having Switches that is limiting in total bandwidth would help save cost. Example you dont need 40Gbps on a 4 Port 10Gbps switch, or a 20Gbps on a 4 Port 5Gbps Switch. For Consumer usage, you only really need to be able to sustain Maximum speed on 1 port, while others working at a lower speed or next speed grade down.
The question is will they get cheaper faster than 10G? Perhaps. Despite being 14 years old 10G is just starting to get put into reasonably priced prosumer switches so it doesn't have that much of a head start practically.