The budget & costs of the project are not the total costs to your firm of
employing the resources but rather that portion of the total cost of those
resources directly attributable to doing the actual work required on the
project. If Sally gets a salary of $2000/month and she spends 40 hours
during the month working on project related activities and 120 hours on
other duties, her cost to the project is $500, not $2000. Because of this,
no matter whether you specify resource costs as daily, monthly, or hourly
rates, Project will track them as an hourly rate times the hours they are
scheduled to work in project tasks.
While there are various ways you might kludge together something to get a
fixed resource cost posted to your plan on a monthly basis, I'd strongly
suggest you not do so because it will seriously distort what your project is
really costing you and your ability to track those costs against your
budget. If you need to work with fixed monthly overhead costs, it would be
far better to track them in Excel rather than in Project as such things
really aren't part of the resource costs to your project in the first place.
