How expensive can be whatever you want it to be. If you want to be independent of others the cost to set up for joining frame materials can be pretty significant. Not worth it for just one frame. With your lack of experience especially it's probably not something you want to consider. Like Andy said, for the hobbyist builder you don't need other expensive tools. And there are ways around the cost of welding equipment too.

Learn to weld first, or at least try. You might find that you just don't have the passion for it.

Right now I have a friend (also a girl) who I'm helping get together her own frame. She has some metal-working skills and is actually a pretty decent mig welder, but has zero experience with torch. So we worked on the design together and she is filing and fitting tubes with a bit of help, but I'll probably finish it up. We'll see how she progresses with the welding practice. This ends up being a win-win for me because I get to continue building frames without the other expenses. Framebuilding is an expensive hobby. The frame is relatively cheap but you got to add the other bits to make it ridable.

I have built a 650C bike. It's fillet brazed but this particular frame could have employed lugs easily. Not totally necessary but with 650C you may want to build the fork too.