There are a couple scenarios I can think of that might be influencing your issue.
The first one is that a get domain or available call will give the base price, which doesn't include the $0.18 (per year) ICANN fee. So if the difference between the price your call returned and the price you paid was $0.18 x number of years registered, that would explain it.
The next is if you're using the Domain API OTE (test environment) to run the availability call. That will return a price that hasn't been updated in several years and is much lower than the current registration price. It doesn't sound like that's what you were doing, but I wanted to mention it just in case.
If neither of those explanations fits your situation, we'd need to know more about the domain you registered and the price difference.
For the sake of security, you may want to be careful not to share any unique identifying information about your account & purchase here, but you can always email firstname.lastname@example.org to have our support team check it out for you.
Angela B - GoDaddy | Community Manager | 24/7 support available at x.co/247support | Remember to choose a solution and give kudos.