It really depends on so many things. If you're numbers are unique and are going work with Twitter, Facebook, Google, Whatsapp etc. If you're planning to charge users for each verification or based on a monthly subscription. There is a service called smsverify.club that charge 0.6$ per verification for example. You will need to make competitive rates and still make it profitable for you.
Indeed, a site i used in the past charged just 0.1$/0.05$ for the verification of google/twitter accounts, but it seems that a lot of their numbers have stopped working/ are blocked by google.
See
http://smspva.com/ if you're interested in looking into the rates of some of your competitors.
I am planning to create a SMS Verification Service with API. What is the best price should I charge per SMS?
I'd be more than happy to pay 0.3-0.5$ for twitter/google if your number would actually work instantly. ( Which is still 3x more expensive than what smspva charges, but i've found their service to be barely working currently.)