It would be nice if the Chinese DSO manufacturers made sampling accessories ... the 10s of GHz sampling ADCs are too expensive for their target market, but a high frequency sampling head doesn't really need to be that expensive.
Of course they can't even be bothered designing active probes, so fat chance.
I agree. A 20GHz sampling head could probably be sold for $50 if it was made in reasonable quantity. I remember reading a Sampling scope designer who claimed that it was not hard to make a 10GHz sampling head with off-the-shelf components (if you know exactly what you are doing). There is skill in the design, but after that, not many components.
Triggering is another matter. The performance of a sampling scope is all about an accurate trigger with jitter in the picosecond range. The modern sampling scopes often have to offer a range of triggering options to cover different situations.
Still, a cheap fast sampling head would be a great start.
There is a simple design that can be used for input frequencies up to about 1GHz from memory. Can't find the article, but here is the circuit. It can be used with any scope with a X-Y view option. Speed is not important.
Edit: Google to the rescue again. By searching for the text in the image, I found the original article which is great - the first time I have seen it.
http://www.redrok.com/Circuits_1GHz-samplig-Oscilloscope-Front-End.pdf300ps risetime, and you only need a 10MHz scope. The RC time constant of the sampling bridge is 100ps.