If a typical 1GHz or more scope costs a lot, and tries to keep a level output across the whole 1GHz, does anyone make a CHEAPER scope that might only be good, for example from 100MHz to 1GHz ??

 

It's the higher frequencies that are difficult to reproduce.
If the scope has flat response at the higher frequencies, then the lower ones are almost a gimme.

And why would you want a scope that's only good above 100MHz?
Are you only interested in high RF frequencies?

 

If a typical 1GHz or more scope costs a lot, and tries to keep a level output across the whole 1GHz, does anyone make a CHEAPER scope that might only be good, for example from 100MHz to 1GHz ??

OK based on that line of thought it seems you are looking for a solution for something.

Instead tell us your real requirements and maybe we can offer recommendations.