Here's a quick list of some of key reasons for the high relative cost of tiny houses, from my own mind and culled from what I've read in comments and elsewhere.
- Foundation costs - even a small slab foundation is likely to run about $8000. Admittedly, some tiny houses are constructed on trailer frames of some sort, but many are also built on permanent foundations, as I understand the ones in this case are.
- Bathroom and kitchen - there's a certain minimal cost involved in purchasing and installing the required fixtures and appliances, and this cost only increases when smaller, less-common versions are used (supply and demand) instead of standard ones.
- Code compliance - while a "mobile home" has to meet building codes, travel trailers and other RV's have looser standard.
- Finish quality - many of the tiny houses are built with pricier materials and finishes than a regular-sized stick-built house, let alone a mobile home. That may not be an issue in this particular story.
- Economies of scale - tiny house construction projects usually lack any economy of scale. They're typically built as one-offs or in very small volumes. Mobile homes are often built more like assembly-line work than normal houses, and even subdivisions of normal stick-built houses can have decent economies of scale with respect to materials ordering, labor, plan costs, etc.