A bit late to chip in, but there are still plenty of used cubs that are going for about £1000,
Heres a typical example that sold on ebay.
www.ebay.co.uk/itm/Honda-C90-Cub-/144788951329
My bike had the same issues as this one, I took the footpegs off and heated them red hot and re bent them in a vice. The paint uner the seat I coated in rust converter. Its fine now.
I think what is happening is that there are people who are restoring these and selling them on, for silly money. There are enough hipster types out there now that will buy things because they are "cool" not really knowing anything about the bikes.
Look for any bike on ebay and you will find ones that are very clean and very expensive. Check the sold prices of Auction bikes vs Buy it Now and you get a real idea of the value. On ebay you can tick various boxes on the left of the screen to just show completed/sold.
I bought mine as a bike to go in a campervan and I went for cheapest one I could find locally with an MOT. My bike is quite tatty and its a high milage, but I like that as I'm less worried about getting it stolen. All the badges have gradually dissapeared off it parking it a few times in a city, I have kept the mechanics ok but the paint is faded, has gaffer tape holding the guard together that sort of thing. Mine might just be worth £1000 but I've used it for years and it always passes its MOTS.
I have used mine for quite long runs eg 2hours on A roads, been up welsh mountains, driven through the Lakes in the winter, and it has never failed me, always starts after 2 or 3 kicks.
The main thing is to check the frame tube is not rusty at the rear wheel, anything else can be fixed fairly easily.
The only thing I wish now is that I bought a 40 year old one as they are Tax and MOT free, that would save time and money each year. But they might be more expensive or rusty!
The parts are cheap so you can gradually do it up f you want, and it should be as good as money in the bank as far as gaining in value with inflation.
Buying a high milage old cub is very different to say a water cooled 4 cylinder bike, parts are way cheaper and easier to find. I'm not saying a bigger bike is a bad idea just economically they are different.
I would avoid the Chinese copies, even in Asia they have a repuation for not being as good as the Hondas.