Whin pricing depends on the plan you choose. Each plan has two main dimensions:

• total number of messages (per day or per month) and
• rate (number of messages per second).

The average user of whin should be able to live with the PRO plan. Free plan is good for testing and/or home Labs DIYers. Ultra and Mega plans are mainly used by power users.

If the cost of using whin is too high for you, Whatsapp (official) cloud api seems like a cheapest option:
pricing: https://developers.facebook.com/docs/whatsapp/updates-to-pricing and https://go.facebookinc.com/rs/267-PVB-941/images/WhatsApp-Business-Platform-Pricing-Explainer.pdf?fbclid=IwAR1SFk-RLMHH-Ogr7jQ4UpacHgRXqKQLTH50RuZk5nlg0we3we2Lb6HLl6s
Although a little bit difficult for us to understand, it looks like it will cost you something below US$ 1,00 per ¿month?.