I think it's on-topic, since the negotiation tag exists, and one of its question (How do I negotiate with a company to develop software?) is similar to the question of mine. However, that question was asked in 2011, and looking up in the What topics can I ask about here?, I'm not sure if my question is on-topic or not. Further more, I'm aware of the proposal of changing the name of the site, and I don't feel like the proposed name best suit for me. After all, I just someone who code, not a qualified professional programmer.
My draft question: I'm a solo "programmer", and I need to buy data for my program. All of the vendors both ask me what is my budget to buy it, and the prediction of the number of users will use my program. How should I negotiate with them?