I want to know if professional org buy REST API & SOAP Web Services usage, but I need to know it from code. It will be great if there are something like "Something.IsApiEnabled()" or "Something.IsFeatureEnable(Something.REST_API)" methods.
There's no API that tells you if the API is active for a given org. You need to make an API call, such as to
/services/data/; if you get a 4xx error, you don't have API access, otherwise you do. Note that you'll need to read the error message to determine why you don't have API access; it may be that your profile does not have API access, or your org does not. The relevant errors are API_CURRENTLY_DISABLED and API_DISABLED_FOR_ORG for user and org restrictions, respectively.