There is a 25% vat tax and then on top of that you have income tax which is around 25% at that income bracket.

25% VAT would mean that 50k paid by the gallery is 30k to the artist, not 25k.

(50k inc VAT is 40k revenue, less 25% of that revenue as tax.)

Correct, sloppy calculation.

Is the income tax net of expenses?

From what I know, If you're self-employed you'll also have to pay payroll tax, which makes taxes end up close to 50% without even taking taking sales taxes into account.