It's sobering that one has to sell about 400 to 500 copies of an eBook to "crowd fund" a professional final edit & proof and professional cover.
Still I'm committed to up dating existing SP eBooks when mistakes are found, or if there are sales to have it done professionally.
But that's not far off what you need to sell to break even for a properly produced self published book, allowing for say, £150-200 for a cover and £400-600 for editing (based on a full edit by a pro editor and a copy edit.) To cover that, you're looking at about 400 sales at around the £2.50 price point on kindle and a £2 profit on paperback.