Now that I've worked out a routine exposing on photo paper and paper positives I feel I have reached my intent of making cheap almost-instant photography.
I can now - when conditions are good - expose & develop + scan a 9x12 paper photograph within 25 minutes. In theory I could print it - i e hold a finished photograph in my hand - within that time (if I had a printer). I think that's rather 'instant'!
The costs involved, per photograph, are as approximated:
almost nothing for 7x11,5 cm very expired photo paper bought in several packs of 100, or €0,3 apiece for 9x12 cm Harman Positive Paper cut into size
almost nothing for developer (rosemary tea or instant coffee base with vitamin C and washing soda)
almost nothing per negative for fixer (Ilford)
Total cost: Almost Nothing.
Of course there is the cost of the scanning equipment if you want decent photos to edit. There are free apps like Filmlab which will reverse your negative photos, so you can see them in positive, though the app is not a good scanning option.
Or, of course, alternative processes like contact printing etc.
Check out more photos on my instagram ourbooksmalmo, or cameras etc. in my Etsy shop getOurBooks.