I have experience of using blockchain and coinbase API for some of my clients and they provide great service. As far as i know they only charge for transaction and they have no monthly charges for API users. Although I don't know they have added monthly charges now or not but their service is quite good.
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Top 10 Best Blockchain APIs. By visiting this link you will get details about top 10 blockchain API and their pricing which may help you to get the best one for your business.