If your business involves jobs or projects, budgeting will probably include aspects of both product and service revenue budgeting. Based on your line of business, you will know what is involved in completing the job or project. The work may involve materials and supplies, direct and indirect labor, use of equipment, and maybe subcontractors. Budgeting for jobs or projects may go hand-in-hand with submitting job estimates or bids. The terms and conditions of the work will play an important role in how the work is quoted or budgeted. A lump sum contract for a particular job is different from a fixed rate contract, for instance.
Breaking down the job into its component parts will be a key factor in submitting a good quote or bid, and budgeting job costs and revenues. You will want to make sure you have all your bases covered, in the sense that you include all your costs, both direct and indirect, and allow yourself a sufficient margin. You may need to make out a materials list, a job schedule for direct labor, and an allocation calculation to include indirect expenses. If you are going to use subcontractors, you should get bids from them, hopefully before committing yourself to the job.
If your business involves jobs that extend over a longer period of time, first you will want to make sure the amount you are charging for the job is sufficient to cover all your expenses and leave you with a margin that gives you an adequate monthly income during the period you are working on the job. You can budget monthly revenue and cost by spreading the total job over the months of work.
If you are working on several jobs at a time, you can use your timelines for each job, and then allocate the total revenues and costs for all the jobs over the applicable periods in order to come up a monthly budget.