Modulars are highly regulated by the government. They must be built to set standards, primarily because at one time there were no regulations and they were of very poor quality which definitely did cause them to lose value fast. Many municipalities either banned them from being put in their jurisdiction, or they specified roof pitches and electrical components that they knew would not allow them. Eventually the Government got into the act and made them toe the line and allow them into areas that previously were closed to Modulars outside of a park. This was also chiefly responsible for parks specifying that the unit had to go if it was a certain age and or had several owners....then out of the park they went.
The current problem is that for so long, the government sponsored programs to buy them have dried up....HUD, VA, FHA, etc....who did nearly all of the financing of these are losing funding....and the government is moving away from backing loans at light speed. They want out of the business.
So there is no funding for them unless you find a local bank or credit union who will fund it more based upon YOU rather than the unit, and even then the unit has to be up to standards, with certifications and the like or they won't do it either.
Many of the modern units are pretty darn good...and are definitely affordable compared to stick built which in some areas don't even measure up to those of the codes for modulars.
Meantime the market for these units continues to go away. Anyone who wants to check out some resale values can review the HUD website and county which they live and see what is happening with modulars there.
http://www.hudhomestore.com/HudHome/Index.aspx
These are HUD repos....and it tells the tale of where these things are going. I just talked to a broker in Mid Michigan who is buying these up after they lay on the market for awhile and is flipping them with land contracts....perhaps you could research some of these guys out and see if they would do a land contract for you on yours...they apparently are finding investors willing to do that.