There is a discussion of the cost advantages of large versus small reactors at thoriumenergy and Left-atomics.
Mainly they are focusing on the traditional discussion of whether the economies of scale of large reactors can be matched by the economies of scale of many duplicate smaller modules. The smaller modules can one control center to avoid unnecessary duplication.
I have covered small modular reactors frequently and looked at the latest large reactors as well.
The main cost advantage of factory produced reactors would be that they could be built in factories in China, Russia or other places like that.
Nuclear reactors built in China cost about $1565/kw.
Russia is also planning for sub-$1500/kw reactors.
The plus or minus 20% of large versus many smaller modules is less than the 300-600% cost differential of a reactor built in the United States or Europe versus a China or Russian built reactor. Small modular reactors allow the cost advantage of China and Russia to be obtained for nuclear reactors, just like the cost advantage of things we buy from Walmart.
The cost of shipping or transporting by rail would be a nominal part. China's reactors are pricing out at $1500/kw, which is about three times or more less than US reactors. The pebble bed HTR-PM reactor that China is making could be 10-20% less or it might initially start out a bit more expensive versus Chinese construction of a LWR/PWR [traditional large reactors]. Russia's smaller lead breeder reactors and other reactors are also supposed to be in the $1500/kw or less range. These new reactors have inherently safer designs. The pebble beds have walk away meltdown resistant designs. These small reactors are also heading towards 2-3 year construction times. Short and predictable construction times reduces the interest and financing charges, which are the majority of the cost of large projects. Smaller modules means that the total debt that must be obtained is less. A utility can buy one reactor module for $50-500 million and then start operating it to get some cashflow going and then build another module. The amount of debt could be controlled to not exceed $50-500 million. For large reactors, a utility might need to arrange for $5-10 billion of debt. China and Russia will likely take the first two dozen of the modular reactors domestically. Eventually the US can certify them and benefit from the lower costs.
So it is a matter of getting the new modular reactors developed and then having them certified for use and installation in the United States.
Some other alternatives are to have floating reactors (Russia is building floating reactors) off the coast and having long power cables into the United States or installing reactors in Mexico and running power across the border.