What free actually means
Most free billing options are one of three things: an open-source project maintained by one or two people, a hosted free tier with a paid ceiling, or a vendor's community edition that leaves the hard parts — payments, integrations, support — on the paid side. All three are legitimate. None of them is free.
The licence costs nothing. The install, the database, the upgrades, and the hours you spend reading forums to fix a broken migration do not. If you have an engineer on staff who enjoys this work, the price is time. If you do not, the price is outages.
Where the free option stops
It stops, almost always, at money movement. Free software can record that a subscriber owes you, but it rarely knows how to take a payment over M-Pesa, match it to the right invoice, and extend the right service — because that work depends on a payment provider's API, a commercial agreement, and someone maintaining it as the provider changes.
It also stops at the network. Synchronising a subscriber's service state with a MikroTik or a RADIUS server is where billing becomes operations: the customer is connected or they are not, and the billing system is the thing that decides. Software without that bridge leaves you typing credentials by hand for every suspension and every reconnection.
The real calculation
Count honestly for a year: setup days, upgrade weekends, the cost of one bad migration, the hours spent on reconciliations that a matching rule would have done, and the revenue lost when a payment path silently broke for a week. That number, against a subscription that includes the payment flow, the router bridge, and support that answers, is the comparison.
If you are a solo operator with forty subscribers, free may win for a while — and that is a fair choice. The calculation changes the moment money is collected every day and people are waiting on your network to be up.
How to choose either way
Start with the questions that are uncomfortable to answer later: who owns the data if you leave, can you export it in a usable form, what happens when the maintainer stops maintaining, and does the tool handle your payment method the way your customers actually pay.
Then decide deliberately, in writing, rather than drifting into it. A billing system is a load-bearing wall. Walls should be chosen, not inherited.