It varies between 5K and 15K per pad, sometimes more in special markets like California. Factors include amount of land being developed, development requirements from the city and county, materials being used, and cost of labor.
If you build a 50 pad Trailer Park in a town on city utilities without zoning, lay down gravel roads, and the going rate for labor is $100 / day you can probably get close to 5K per pad. On the other hand if you're building a 50 pad Mobile Home Court in town that requires pouring concrete pads, driveways, and roads, requires installing fire hydrants, storm sewer lines, meet detention requirements, has density requirements, and you want to pimp it out with gated lots, special lighting, etc, you will probably be closer to 12 or 15K.
These are costs without the homes. Have I developed a MHP from scratch? No. This reply is a tailored regurgitation from the MHU study materials along with my experience developing single lots in small towns with mobile homes.