I agree on the content of the article by Nick Inman, but I have a complementary approach.
From the price that a craftsman would charge me, I deduct the state refund (crédit d’impôt) if applicable and the price of raw materials.
I divide the result by the number of hours I estimate I need to do the job myself. It gives me a price per hour.
On the one hand it gives me the priorities to save money, and on the other, under a certain threshold I entrust the work to a professional.
Note: VAT could be different if the job is done by a craftsman.
Typical example is replacing the windows with double-glazed ones. VAT is 5.5% if the craftman is RGE-certified. It is 20% if you do it yourself.
J-Y Renaud, Loire-Atlantique