I'd say that a hobby by definition costs money. A business can go either way. How much you want to invest can vary significantly. A nice jig and TIG welder can cost you 8k. A torch and homemade basic fixtures $400. And a whole spectrum in between. Same for the rest of the equipment. The lesser variation is in the building materials.

It's hard to say how many frames one needs to make to get passed the steeper part of the learning curve. There is only one way to know... Building one frame under supervision is a great way to get started.