Mobilization is usually bid as a lump-sum line item or as a percentage (2-5%) of the contract value. Demobilization is the matching line item for project closeout: removing temporary infrastructure, cleaning, and final inspection support.
Costs included: equipment transportation to and from the site, jobsite trailer setup, temporary power and water connections, perimeter fencing, project signage, initial materials laydown, project superintendent's setup time, builder's risk insurance setup.
On a public bid, the owner typically caps mobilization at 5% of contract value to prevent front-loading the schedule of values. Watch the cap when sizing the line item in your bid math.