Prices seem to vary wildly.. Pre-lockdown you'd see earlier models going for this sort of money on
ebay; as it happens I've just had a look and see a decent looking one in Edinburgh sold for £450 although most have gone for more. They do generally seem to retain their value well..
Before I bought my 30 I was tempted by a 20 as De Ver Cycles had the 2019 model (same as 2020 offering) at a grand; which seemed like a good buy when most of the used, more recent CdF 20s seemed to be going for ballpark £700 as you suggest. Iin the end I talked myself up to spending the extra on the 30.
The models vary quite a lot from their inception in 2011(?) to present (a frame geometry change around 2015 being a big one) and I did a lot of research before I jumped in, so if you want to know anything about spec / age tag me with a question and I can probably answer it