Start with your operator's rate for the SIM you already own, on the package you are already paying for. In most markets a business SIM with a bundle brings the marginal cost of a message close to nothing until the bundle runs out.
Then add what the alternative costs: a provider's per-message fee, the credit that expires unused, and the minimum spend that unlocks the good rate. Those three together are usually a multiple of the airtime itself.
Finally, count the messages you do not send because of the cost. Delivery confirmations, appointment reminders and follow-ups get cut first, and they are the ones that quietly earn their keep.