Okay yes it makes a bit harder, I have tried to find sub 1GHz probe designs and I have also found some, but I do not know how good they are. If you look at the design I have chosen here, it should work fine up to about 800MHz but only in super low voltage. The design you can buy are for the most for high voltage and yes costs 4 times my scope.
I (and ogden) have already told you this is an enormously difficult task, and your only hope - hell, my only hope! - would be to copy someone else's design exactly. I mean, the same components, board layout, physical construction, every last detail.
This is not the sort of problem a beginner is likely to run into, nor even the sort of problem any but RF and high-speed digital design engineers will ever have to contend with, frankly, so I have to ask why you think you need a >500MHz differential probe? Earlier you wrote this:
...If that had been a 500MHz would that have been enough? I mean, if all I want is to be able to measure where there is different grounds....
But that isn't an answer. Suffice it to say that there are precious few instances where an RF or high speed digital signal is not supplied by an isolated power supply, so there isn't a safety requirement for needing a differential or isolated probe. There are plenty of examples of high speed differential digital signal standards such as Ethernet, LVDS, HDMI, SCSI, etc., but, again, not the sort of thing the typical EE will ever work on at the signal level, much less a beginner or hobbyist.
So what, exactly, do you intend to probe with an oscilloscope that has convinced you you need a really expensive differential probe good for (much greater than) 500MHz?