A construction company might need a mobile office for six months while building a subdivision or a school district might need one for the entire school year. Buying a mobile office trailer is a major investment. A mobile office that costs $200 to $500 per month to rent could retail for $20,000 to $30,000.The cost of leasing that office for a year would be $3,600 to $4,500, not including delivery, setup and removal.So, unless you anticipate using the mobile office trailer for many years or moving it from site to site, leasing is almost always more economical.here is an average flat fee cost of $500 to $800 or more for the skirt. Large trailers often come equipped with bathrooms, which is particularly handy if they’re located on a remote site or the mobile office is a long walk from the main building.