This.
We stayed in a small AirBnB prefab house a few years ago. It was less than a year old at the time and they have come a long way since the "trailer park" days that come to mind when I hear prebuilt house. The biggest issue is probably going to be insulation, where it will burn more energy to heat/cool.
The internet says delivery is $50 to $100 per sq ft. There are prefabs that look fancier than the following site too but, of course, cost more.
Small Edit: If you have local ordinance or HoA agreements that may cause issues, you could look into building a "barn/shop" with an apartment snuck into it. Look up "barndominium" for ideas on that.
The prefab AirBnb somewhat looked like this but probably 800 sq ft instead of this 600 sq ft (interior was the same wood walls/floor). Their site lists 600 sq ft houses at around $80k:
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Our Lelands Cabins series include The Lodge, The Ponderosa, Lone Star, Settlement Harbor, Smokey Bear, The Brazos, The Stag, Chisholm Trail, and Rio Bravo.
www.lelandscabins.com