But this is quite confusing to me – summing of planned and actual costs together – and I assume that that’s the case with other users as well judging by this thread.
I remember another thread where some members asked for project budgets and I managed to get this 
using a quick search:
If we can agree that POs are just planned costs until they’re actually invoiced, then maybe we can summarize all orders in a separate column called “budget” and call it a day.
To take it one step further, I think that a hybrid approach could be best suited since I acknowledge that there are some benefits to the method you propose @lubos. This method has three columns summarizing different aspects of cost:
| Summary Column | Includes | Justification |
|---|---|---|
| Planned cost | POs, SOs | Purely for budgeting purposes |
| Incurred cost | GL entries referencing project | Purely for tracking what has actually been incurred |
| Revised cost | POs, SOs + GL entries - double counting | To refine the plans in light of incurred costs that hasn’t been budgeted |
This looks to me like a win-win situation.