@Graham-Keellings Hi Graham, I have found that you can buy the same screen that onion are using in the OLED expansion and just plug it in directly yourself. I'm not sure how cheap you want but you can get them for 4.5 on Amazon and cheaper on Ebay eg here: https://www.amazon.co.uk/gp/product/B0746JRHSM/ref=oh_aui_search_detailpage?ie=UTF8&psc=1
You can address it just using the I2C connections and power from the Onion. I keep meaning to write a quick howto on it...
I did see someone else's page about this, that I should reference also.
I plugged it into a dock here when I was trying it out but it can connect just to the pins on the onion.
While on the conversation of making things cheaply, I have also had some adaptors printed in china to breakout the pin pitch size up from 2.00 to 2.54mm so that they are easier to stick into breadboards or stripboards etc. I think they cost about 24 pence each? Makes using the Onion much more affordable.
Again, I keep meaning to write up some how-to for all this too :-)