Many are made from shipping containers.
Some are here, Siminized for Sims 3.
http://www.cheapcontainerhomes.com.au/
They are great starter homes, in RL Australia.
The first problem with these plans is that while humans can climb across to the other side of a double bed, which is against a wall, Sims can't. So I thought about making the bedrooms into singles, keeping the dimensions of the container-house the same. You may also need to rearrange bathroom furniture, but the space will work.
The first fits on a 15 x 10 lot.
The first Sim to move into one of these, in Bridgeport, was Jeromy Craig.
Cost: §10,056
The next home was occupied by the Bunmui twins. On a 30 x 15 lot. 2 bedrooms, 2 bathrooms.
Cost: §20,089
The third home. 3 bedrooms, 2 bathrooms.
Alvin Dutch, married to Sarah Ferguson, teen daughters Cadence and Breanne.
Download the house. On a 25 x 15 lot, so a little cheaper, but with a pleasant decorative garden feature.
Cost: §17,926
Sita January 6, 2017