A recognized artist at a mid-high level gallery is usually promised a show approx. every 18 months. Average earnings for such a show is commonly expected to be around $100.000, (10-15 works each selling for between $5.000 and $10.000).
That would be a decent amount of money for that time but here comes the crux; the gallery takes 50%.
This leaves the artist with an income of $50.000 before taxes. Say you live in Sweden, where you have to tax nearly 50% for sales in a private company that leaves you with $25.000 every 18 months in disposable income, hardly something you can get a flat in Stockholm for.
What's even worse is that if you are tied to a gallery you are most commonly prohibited to sell privately or show with other galleries.