I looked over some of the ones we have to consider for work later this year, all from charpentiers and menuisiers, the estimates are quite amazingly far apart in most cases. Anyway, a couple of them have a clause of the kind you say and one is very specific about price hikes and raised delivery charges being taken into account but itemised by him in the event. The ones to look out for are more those who ask for a reasonable advance, bulk of the money during the work and a tiny percentage on completion. As more than one person who knows has warned, that helps them walk away from an uncompleted job with only a little due that if anybody took action and they had to finish they would then get. However, as two artisans have said, a few sly buggers estimate that little final payment in the advance and bulk knowing they can walk away and how hard it is to then get them to finish the work. The local menuisier who we know very well anyway said that more than a few of them know that it will make some work for somebody else who may well find something that needs fixing. As like as not the second one in knows the first, will knock him like hell face to face with you and then laugh about it over a beer. Properly qualified woodworkers who are members of the Chambre des Professionnels du Bois do not do that because they are penalised and, as he says, other trades in their associations likewise. Use people who are chambre de métiers members to lower the risks all round. The devis we have had that are dodgy are on the one hand two foreign and one French artisans and on the other not chambre members, they have all been dishonest about being able to supply work for which we can benefit from crédit d'impôt although the department does not list their names which we check after we have the devis.
So, all round, in reality if people want to be thorough they need to dig a bit. However, despite the difference in estimates we have had, the people are generally very precise and seem only to have different ideas about one tricky bit of the job where, my gut feeling tells me anyway, probably the one who asked the highest price actually has the best solution. So, coming from a Scot this might be strange, but do not try to save by taking the lowest quote without careful consideration. It costs an awful lot to put right what was done badly on the cheap.