Is estapro.org a legitimate agent for ESTA applications? [duplicate]

No, the only official ESTA form is https://esta.cbp.dhs.gov. When talking about the US, you should be wary of official-looking sites that don't end in .gov.

As far as I understand it, the business model of those sites is just to rip you off (charging you 60€ instead of ~$15). I wouldn't be tooo concerned about them abusing your information.

While this is clearly not the official ESTA website and their business model may be questionable, I believe that this is a legitimate business website and I see no reason to worry that your personal data may be used illegally. If you're concerned that you've given them your consent to process your personal data in a way you wouldn't like, you can write to the e-mail address listed on their privacy policy page and revoke your consent:

Revocation of your consent to data processing

Some data processing procedures are only possible with your express consent. A revocation of your already given consent is possible at any time. An informal notification by e-mail is sufficient for the revocation. The legality of the data processing up to the revocation remains unaffected by the revocation.