It's like anything else. As you spend more you will realize certain improvements in the quality of the bike and the benefits it provides. After a certain point, the magnitude of the return diminishes. There is not a whole lot of difference between the bike I spent $4k on and the bike I spent $1200 on within a few months of each other. But, for me there is enjoyment and benefit in having both. It all depends on what one is looking for.

It's a lot like cars. As you go from Volkswagen to Audi to Porsche, you will see improvements in various areas of the cars from model to model and brand to brand. For some, there is great joy in owning a car that costs over, and sometimes well over, $100k. For others, they are content with the lowest price car that gets them from point A to point B.

If your goal is just exercise, then any bike would give you that if you use it. If you want to go farther, faster, and have other benefits from a bike, then one tailored to what you want to do may give you more enjoyment and be worth spending more money on.