Hi @Clinton_P, this is a great set of questions. Thanks for asking them!
I agree with what @Ronald_Munjoma has said. ODK software itself is available for no cost but using ODK is rarely free. The software has to run on servers which cost money. And it will require someone's time (and thus money) to build forms, collect the data, and do the analysis. The latter is what you are providing as a service and it's fair to charge for your time and the additional value you are providing.
Please note that ODK Aggregate is no longer being updated. We recommend you use ODK Central instead. Central's submissions are available as an OData feed that you can connect Excel, Power BI, Google Data Studio, R, or your Python scripts to. If you need to interact with more than the submission data, Central also has a REST API that is a joy to work with in Python. See this example I recently posted.
You can host Central yourself (just like you could with Aggregate), but the easiest way to use Central is on ODK Cloud, where we do all technical work of running it so you can focus on your data. ODK Cloud also includes incredibly fast support from the core team.
I mention ODK Cloud because in the same way that you should be compensated for the value you provide, we think the team that does most of the work on ODK should also be compensated for the work we do. ODK Cloud is what pays for the core team's time.
We understand that not everyone has budget to pay for cloud hosting or premium support, so we make the software available at no cost and run this forum in the hopes that those who don't have budget can contribute in other ways. In your case, perhaps you can help answer support questions, improve translations in Central, provide feedback on the latest Collect beta, and of course, encourage your clients to use ODK Cloud.
In my experience, not every client cares that something is open-source, but most care about the value they are getting and want to ensure they aren't locked into a platform. With ODK, they are using a platform that is proven at global scale, trusted across sectors, and community-oriented open-source. They can use ODK as is, participate in its improvement, or try compatible alternatives. Those are huge benefits that you should certainly tell your client about!