It really depends on a number of things. If you looking for a formula you won't really find one that fits every business. It really depends on the strategic direction of your organization. Let you goals and objectives drive your budgeting weights. If your goal is to sustain a product it also is different at every business. For example- a poorly developed product may require more staff than the same product that is of better quality.
I had to do a similar activity when my company went on a hiring spree to developer a product and when it was complete we tried to maintain the development team but it just wasn't possible. I spent weeks try to identify the lowest possible operating costs to sustain existing service levels while reducing staff. The most effective way I found to identify the lowest possible operating costs was to really find out what everyone was working on. I analyzed time sheets, support tickets and backlogs then I categorized the needs from wants.
Hope this helps...