You will also find that not all hatmakers use the terminology the same, so one guy's 4X silverbelly may not be the same as the next guy's. (E.g., note Dave's comment about "100%...now called 30X" or similar.
About how much to spend, it's a personal choice depending on a lot of things -- not least of which how much coin you want to drop. Here's one way of looking at it: If the hat is a novelty or a decoration that you will rarely wear, maybe $100 or $200 is as high as you should go. If you are gonna wear it daily, and can afford it, a custom hat is definitely the way to go. They fit perfectly and you have a choice of high quality materials, shapes, etc.
I don't wear cowboy hats, but I do wear a fedora or a panama near daily. The custom fedora that I use for business cost about $500, about what I pay for a good semicustom suit. My best panama, a Montecristi, more. My old knock around, off-the-shelf-fedora cost ~$250 about 15 years back, but at this point I probably will replace it.
(There are also panama hat collectors who spend thousands per hat and never wear them, but just keep them as art on display. There are only a few folks left in Equador who make the genuine Montecristi, and they spend months per hat. Very, very fine weave, etc.)
If I were you, I would read up on hats in general, and cowboy hats specifically. Maybe join a hat forum. (But then I am a hat guy, and enjoy the learning process.)
Tip for those of you with old hats that have shrunk: The guys who clean and block can resize 'em back to their original size. You can also buy a hat stretcher which works, though not as well. (Took me about ten years to figure that out!

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