You don't say where you want to build the house. However, here's one example of a modular home product; these homes are manufactured in plants in Indiana, Ohio, N. Carolina, Colorado, and Iowa.
These are not double-wides. They are set with a crane on either a basement or crawl space. Basically they are "stick-built" in a factory instead of at the house site.
You may look at floor plans on the website. The standard product options of each model include cabinets, flooring, plumbing fixtures, baseboard heat, light fixtures, siding, trim, etc. However it is possible to order the house and eliminate some things such as flooring or cabinets or baseboard heat, etc. if the customer wants something different or wishes to install their own. However, if the options supplied are suitable, it is usually cost-effective to go ahead and take the factory-installed options. I believe they now supply garage modules as well.
Pricing is so dependent on such things as options ordered, distance from the plant, local costs, building codes, utility hookups, well, septic if needed, and site preparation, that it is necessary to get price quotes from an authorized dealer and bids from local subs for site work. There are just too many variables. The manufacturer that I have cited does not sell their houses direct. Price quotes can be obtained from one of their builder/dealers. Yes, "sweat" equity is certainly possible with these houses. Handy people have done some of their own work, using whatever skills they happen to have - be it excavation, cement, building the garage, painting, installing gutters & downspouts, carpeting, even finishing an unfinished second floor (one of the capes can be ordered with an unfinished second story).
We built a cape cod style modular home and love it. We did not do much of our own work however. I can see how one might save a few dollars doing some things yourself. The best way to tell how much you could save (remember that your time is also worth something!) would be to get a quote on all the work that needs to be done + a quote on the modular portion and add them together. Then start subtracting the labor costs from the jobs that you might be able to do yourself. Your dealer could help here, suggesting site and finish work that you might do. With the particular manufacturer that I have cited above, the cost of the house includes delivery and "rough" set. The "finish" set is done by a local crew who is knowledgeable about what is required with the finish of a modular house. The finish is usually not something that the homeowner would want to tackle by himself and is best left to others.