Are all the numbers used for OTP or can the number be re-used? will be great if you add reused numbers.
It will be difficult to trust it in some accounts, such as Google accounts, protonmail, and Microsoft, so it is better to provide it for some public services like whatsapp,...etc.
Prices start at $0.50 per code, changes depending on the service and the country.
You need to add more information about the countries in which you can provide the service, and the prices for each country.
Some services, like protonmail, only ask for SMS verification as an anti-spam and not to add as a recovery method (two separate options). Account security is not impacted.
Prices are dynamic, I have numbers from over 90 countries and over 30 services per country so I'd rather stick to the "request a quote" system since it also depends on the amount. In any case it's always between 50 cents and 1.5$ or so.