How can I create a marketplace offer without prices / plans?

Thank you for your question.

Microsoft's commercial marketplace allows for listing and transacting many different https://learn.microsoft.com/en-us/partner-center/marketplace-offers/determine-your-listing-type (SaaS, Virtual Machines, Containers, Azure Apps, and Professional Services). Additionally, Microsoft partners have options on pricing models when it comes to creating plans for their products (referred to as "Offers"). See https://learn.microsoft.com/en-us/partner-center/marketplace-offers/plans-pricing 

Since you mention your pricing structure is often negotiated directly with each customer, I'd encourage you to learn more about the benefits of selling to Azure customers with Private offers. For a plan to be eligible, it must be public and have a price published to the marketplace, empowering your organization to https://www.microsoft.com/en-us/isv/marketplace

Many software providers establish their public plans with base prices and then https://learn.microsoft.com/en-us/partner-center/marketplace-offers/isv-customer for each customer to meet the unique negotiations such as custom pricing, terms and conditions, and dates by when the private offer must be accepted and the product purchased.

If you prefer to not allow for customers to purchase your solution "off the shelf" in a self-serve way, and you also do not wish to publish any pricing, you can opt to transact with customers by creating private plans, however, you will be limited to less than 50 transactions as there are caps to the number of private plans you can create. Microsoft continues to invest in new capabilities for the Private Offer sales motion preferred by many partners and customers. You can https://learn.microsoft.com/en-us/partner-center/marketplace-offers/isv-customer-faq to understand the differences. 

Hopefully these resources help you confidently publish your offer to the marketplace so you can begin to reach more customers and accelerate growth through the Microsoft ecosystem. Let me know if you have additional questions. 

-Kyle