I did / do think that most electronic, made by Agilent and other is much more expencive then needed to be, due to name, manufacturing and the 1000V capabilities.
Urban myth. If you have ANY 500MHz "original/brand" probe, then buy cheap 500MHz probe from China (if any) and compare. You will see that "you get what you pay for" saying makes sense.
So I did thought, DIY and low voltage (65V) would be much cheaper and maybe not as difficult as a 2GHz 1000V Agilent you can buy.
Dreamer 
65V differential probe that compares to 2GHz 100V Agilent probe may take years of your "career" and all your savings. Better just buy one.
[edit] Failure to complete my 10 minute homework even after so many posts just confirms that probe you are dreaming about is above your paygrade. Sorry.
It makes a lot of sense, my thought just comes from so many other things you can buy that alone is expensive due to brand.
I think that most probes are above my paygrade, even the "cheap" one:-)
LOL I love the way you say it about not building a space rocket before even learn to fold a paper plane:-) But again as a beginner, it is hard to know what is a paper plane project and what is a moon landing:-)
Not really! Just create a spreadsheet of specs and price. Specs would include Common Mode (AC+DC) Max Voltage, Differential Max Voltage, Switchable Voltage Ranges, Bandwidth and so on. Maybe one column for each.
You might include a column for Safety Certifications and another for Warranty.
For voltages less than 50V (peak to ground), I might not get too concerned about safety certs but beyond that, where voltages become life threatening, I would have a lot more concern. That's why I keep my projects at of below +-15V.
What a fantastic idea, thanks, it "forces" me also to dig deeper in what I am about to jump into, so that is a thing I will do from today and on! :-)
Yes above 50V is some nasty stuff and can even be dangerous already from 40V or even lower if all things goes wrong!
I did / do think that most electronic, made by Agilent and other is much more expencive then needed to be, due to name, manufacturing and the 1000V capabilities. So I did thought, DIY and low voltage (65V) would be much cheaper and maybe not as difficult as a 2GHz 1000V Agilent you can buy.
Don't forget the cast of dozens that worked on the design, the management that lead the project and up the chain. Then there is the cost of safety certifications and the ever important QA department. Those are substantial costs to be recovered from what is bound to be a low volume device.
It's easy to build a Rigol DS1054Z and sell it for $400 when they are going to sell millions of units. I doubt that Keysight has many customers for their $300,000 model. I rather expect it is built to order.
The EEVblog store has Dave's differential probe for sale for A$350 but the bandwidth is just 70 MHz. It has a voltage range switch for dealing with up to 700V differential.
https://www.eevblog.com/product/hvp70/
Sapphire makes Dave's probe and many others that get rebadged by the scope manufacturers. Look at the list here and see if what you need is even manufactured. For the higher voltages, 100 MHz looks like the limit.
https://www.nbn.cz/Sapphire/Sapphire%20Instruments%20CO_%20LTD%20produts_files/pproducts.htm
Yes I have looked at these also but the price did frighten me and then did I think: "why not make it myself, it is a few components and I do not require GHz+ or high voltage, so even cheaper" LOL was I wrong:-)
My reason for talking about differential probe, is that, that's what I did find out after several weeks of searching, but is it what I need?
I "just" want to be able to measure two places where the ground is not the same or where I am not sure if a common ground connection will give me a wrong result.
EDIT: Sorry for this maybe infantile addition but I imagine some circuit where it measures the difference between ground and probe point, send that voltage out to the scope, so lets say DC difference of 15V gives the scope a 10:1 or 1.5V or AC 2VP to P sends 0.2V to the scope.