Free software is free. The stack around it isn't, and the price shows up in three places that are easy to miss because none of them arrive as an invoice.
Time. Manual entry and reconciliation across two or three tools is a few hours a week that you're not spending on production, sales, or rest.
Errors that compound. A spreadsheet mistake doesn't announce itself. It sits in your cost per unit, and every pricing decision you make after that inherits it.
Underpricing. This is the expensive one. If your cost per unit is a number you estimated once rather than one that updates when your supplier raises prices, you are almost certainly charging too little for something — and the products you sell most are the ones where being wrong costs the most.
That's the actual argument for paying for software, and it's worth being precise about it: not that spreadsheets are bad, but that a wrong number gets baked into your pricing, your wholesale agreements, and your reorder quantities — and by the time you notice, those decisions are load-bearing.