A separate argument covers why a business case built only on financials tends to fail. This one assumes you accept that and still have to produce the numbers.
Source to Pay is unusual in that you genuinely can. Both the costs and the benefits are measurable and largely predictable — which is exactly why an unsubstantiated ROI figure is so damaging in this category. The data was available and you chose not to gather it.
Part one: what it costs you today
Two components, both built bottom-up.
People time. Map the existing process, then work out the time each task takes and which roles do it. You need a fully loaded cost per hour for each role — salary plus on-costs, not the salary line alone.
Infrastructure and software. Every piece of technology in the current process, with its direct costs (licences) and its indirect ones (platform support, the internal time spent keeping it running).
Part two: the spend analysis nobody does
This is the most commonly skipped part of a Procure to Pay business case and usually the most valuable.
Most organisations have accounting-level information for each transaction and believe that constitutes spend visibility. It does not. Think about what hides inside a general ledger line called "Administration" — the detail that decides where savings actually are is precisely the detail that line conceals.
A useful spend analysis answers one question: where are we spending our money? To answer it properly it should:
- Cover at least 80% of spend by dollar value
- Use extracted data for the last twelve months
- Be mapped by supplier, and by business unit or location
- Identify the commodity each transaction is for — a combination of GL account code and spend description usually gets you there
- Separate contracted from non-contracted spend at transaction level, not supplier level, because a supplier with a contract still generates off-contract spend
The output tells you where the money goes, to whom, on what, and whether it is managed. Everything in the benefit case depends on it.
Part three: what the future costs
Total cost of ownership over at least three years, aligned to a future process map and transformation roadmap so the timeline and resourcing are explicit rather than assumed. Include:
- Software
- Change management
- User training and support
- Integration
- External implementation
- Operating model change — new roles, new salaries
That last one is routinely left out, and it is often the difference between a case that survives year two and one that quietly stops being referenced.
The benefits, in three categories
Procurement benefits
These come from systematic control over spend and from the insight the spend analysis produced. With commodity-level data you can identify where to consolidate suppliers, which high-value commodities sit outside any contract, and which contracts fall due for renewal in the next three years — renegotiated by a team that now has real data.
Our own research puts potential cost savings up to 15% in some spending categories. Applying that number uniformly is how business cases lose credibility, so band it by how confident you actually are, category by category:
- Low confidence — 1–2% cost savings
- Medium confidence — 3–5%
- High confidence — 6–8%
A case that shows its confidence bands is far harder to argue with than one that applies a single optimistic percentage to everything.
Invoice processing benefits
Three levers reduce what processing an invoice costs:
- Automation removes manual handling through eInvoicing, purchase order matching and fraud detection.
- Digitised approval workflows — pre-approving spend via purchase orders makes most invoice approval automatic, with exceptions routed rather than chased.
- Process accountability — visibility across the whole workflow lets you baseline performance and improve it, which is a compounding benefit rather than a one-off.
Calculate it the same way as the as-is: assign time and roles to the future process map using the loaded salary figures you already built, then subtract from the current cost. Same method, same rigour, comparable number.
People benefits
The most speculative, and worth including honestly rather than inflating.
The premise is straightforward: what is worth more, staff doing administrative work or value-generating work? If front-line staff or managers currently spend two hours a week on admin supporting the Procure to Pay process — measurable from the process mapping you have already done — and the new process drops that to half an hour, what does the recovered time and a half produce?
Label these as the estimates they are. A case that distinguishes between measured and estimated benefits is more persuasive than one that presents both with the same confidence, because the reader can see you know the difference.
The whole point
Every number above traces back to something concrete: a process map, an extract, a salary, a contract renewal date. That is what makes the case defensible under questioning, and it is what makes the project deliverable afterwards — because the same baseline that justified it is the one you measure against.
How we deliver Source to Pay → · Why financials alone are not a case →