You should have an answer to the question “How will you make money?” early on. You may even have several answers. It needs to be plausible. People (like investors) may push back and argue with you about whether or not it’s a feasible business model. If you’re asking people for money, it’s a question you will have to deal with. So, you better be prepared for it.
That being said, you pointed out a few important things. For one, it’s okay to not be sure which will be the ideal business model or price. The process of getting to profitability is something you’ll have to face eventually if your startup continues to grow. You may be able to push it off for a while in favor of focusing on growing usage. That’s the second point, if your product is growing quickly, you’ll often find investors willing to fund your growth despite the lack of a proven business model.
There are only a few major business models though: Advertising, Subscription, E-commerce, Business Development, and Lead Gen are some of the major ones.
Let’s take Facebook as an example. In the early days, Facebook was growing so fast that they were able to get a ton of money before they had to worry about their business model. But it was pretty clear their business model was going to be advertising. It’s a fairly straightforward path to monetization for a social network. Though not all social networks monetize solely through advertising (LinkedIn charges users for premium accounts).
There are some others (like Medium) where the business model is still unclear, but I bet that the founders have a path (or several) towards monetization in their heads.
Yes, solving a problem should be the most important thing for you to focus on. But the reality is that if you’re trying build a big business, you have to have an idea how it’s going to be a lucrative problem to solve.