A reporter once shouted out to the famous financier J.P. Morgan "Mr. Morgan, what will the stock market do?" Morgan looked at him, and said "Fluctuate". The same applies to prefabricated homes, and, indeed, to any type of real estate. Whether their value will increase or decrease over a given period of time is a function of what happens to market supply and demand (and given that supply is generally fixed over the short term, given the time it takes to go from the idea for a house to the final Certificate of Occupancy, that means what happens to demand).
Having said that, you have to be careful when talking about homes other than those built on site, as the terminology sometimes gets used loosely. By prefabricated home, I assume you are talking about a home which is constructed in modules in a factory, shipped to the site, and linked together there on a site-built foundation, not a euphemism for what used to be called a trailer, then became a mobile home, and is now, I believe, referred to as a "modular home". Once the true prefabricated home has been assembled on site, it can be virtually impossible to distinguish it from its site-built neighbors. Therefore, if the values of the homes in the area in general go up, the value of the prefabricated home should as well.
You should understand that like your question, this answer is dealing in very broad generalities. Every home is unique, even in a Levittown-style tract deveolopment. You will never really know if your home value has appreciated until you, as seller, walk away from the closing table with a check in your hands.