Seems more like to be feature-related than money-related. You could use the official Facebook Messenger iPhone app which does not have ads and does not cost anything. Or you could open a WebView to
https://m.facebook.com/messages and remove the top bar. But the XMPP gateway inhibits features like read receipts, cash transfers, and Messenger apps, and requires additional supporting software for features like group chats, stickers, photos, voice messages...