Since Dialogflow doesn't provide UI to show your chatbot and user's responses, you will have to use the chat SDK platform that supports integration with Dialogflow.
I agree with paying for the SDK may difficult when you are in the initial stage of your business but looks like it is a bit difficult to check for the platform that provides the free SDK integration.
To render your Dialogflow bot responses on your iOS application, you can use a platform like Kommunicate to add the chat option into your iOS app and this will helps you to add your chatbot into your mobile applications.
To do the integration on your iOS app you can check out this documentation and the blog.