How can I test payment models for an app without having a company to get an account with payment processors such as Stripe?

If you're asking about processing point-of-sale payments, have you considered using the smart phone tools available from Square, PayPal or QuickBooks? You don't need to have a formal business entity structure to use these. A sole proprietorship is a viable business model to conduct some sales and test payments.

Depending on the size of your test you could always establish a "TEST" account with a gateway/payments provider that understands this is a primary step for your proof of concept.

If I understand your question correctly, you could test with Stripe or PayPal without any extensive development, cost, or commitment. Otherwise, you could just use a secured form to test conversion.

To be honest, if your product targets the high value use case, people will pay whatever you charge. http://avc.com/2015/07/bootstrap-your-network-with-a-high-value-niche-use-case/

The other thing you could do is look at competitors and see what they charge, and how are they missing out.