If you're talking about land in a mobile home park, there really is no such per-square-foot calculation that's generally applicable to space lease. Fees can vary widely from park to park, even for similar size lots. I'm not aware of any database of manufactured home parks that gives listings of individual lot sizes along with a report of the space rent. Therefore, it's virtually impossible to come up with the number you're looking for. However, an individual park owner would have the information needed to make that calculation for the lots in his/her own park, if he was so inclined.
If you're talking about land that's not in a mobile home park, it would be even more difficult to come up with a per-square-foot number because almost no one who owns land outside of a mobile home park is willing to rent that land for someone else to put a mobile/manufactured home on. Thus, there is no reasonably-accessible collection of lots (e.g., lots under the same ownership) where a single source can be contacted for lot sizes and lease rates.