A mid range oscilloscope can easily cost $5000. If you look at the whole range of oscilloscopes, $5000 is on the very low end. If you want bespoke, as in custom tailored to your exact specifications, that's easily another order of magnitude in price, possibly two.

Heck, there are oscilloscope probes that cost $5000.

But test equipment isn't exactly high volume stuff.

What call would one have for a bespoke oscilloscope? Surely the whole point of test equipment is that it's standardised?

I'm not sure what you mean here... you want test equipment to be "standard" in the sense that they all agree what 1.0 volt is, but different people can need radically different feature sets from test equipment, as evidenced by the fact that Agilent makes one hundred and fifty three different kinds of oscilloscope, not counting the spectrum analyzers, logic analyzers, signal analyzers, frequency counters...

I can understand wanting more channels or a faster sample rate. And maybe a portable/handheld version. But what other differences are there?

- Ability to do some scripting, or integrate it into a product/project.

- Webserver/standalone operation.

- Integration with other standard comm buses.

- Option for Better A/D (typical o'scope is 8 bits).

- More/better output options. You could potentially use it to simulate some hardware (in the loop).

- Ability to add COTS hardware.

- Open design insures owner against planned obsolescence.

This is kind of like asking why there's more than one model of car on the road. Other than wanting to carry more people or go faster, what differences are there? Any automaker could just produce three vehicles: a sports coupe, a panel truck, and a bus. Maybe a motorcycle (again, just one model).

Depth of the sample buffer. Tradeoffs between sample speed and sample resolution. Triggering. Math (anything from sums/differences to FFTs). Bandwidth, including tricks like equivalent-time sampling. Various kinds of display. Various kinds of human interface. Various kinds of machine interface. Extra features like waveform generators.

Honestly you can figure this out yourself: go to some scope makers' websites (Tektronix, Agilent, B&K, Fluke, Instek, Teledyne Lecroy, etc etc) and see how their marketing department distinguishes their product lines.