For driving schools, a common package includes unlimited self-study sessions for the theoretical exam, 20 one-hour driving lessons and the right to take the practical exam with the school's instructor. Between the driving lessons and the practical exam itself, you have to drive a certain number of kilometers with a friend or relative, in a regular car (that's called “conduite accompagnée” for youngsters or “conduite supervisée” for adults). That's how I learned to drive many years ago and it still seems to work that way.
To give you an idea of current prices, such a package can typically be had for about €800 to €1000 but it can apparently cost up to €1500 in Paris. It's less common nowadays but you can also get a “traditional training” with more time with an instructor and no accompanied driving but the price is higher. Some schools also have an “express training” offer if you need a license ASAP, at a premium (importantly, they can give you an exam spot they already booked a long time ago, thus going around the waiting list).
Also, it seems that 20 hours of training with an instructor is a legal minimum to start the accompanied driving phase and/or register for the exam. Some people actually need more (especially if they fail the practical exam the first time) and then you pay by the hour. But since you already know how to drive, you probably don't need all that. It does seem possible to get a waiver from this requirement (presumably from the préfecture) but it's not completely clear to me how you should go about that.
Because the prices are quite steep, some “low-cost driving school” made the news a few years ago like permispascher.fr. The same website mentions a “pack candidat libre” (to assist you in taking the exam independently) for €350. This includes training material for the theoretical exam and two lessons but you have to take care of the paperwork. You also have to pay €99 if you want to get an exam car from them and probably bring a qualified experienced driver to the exam (see below for more on that). There are also exam packages for about €250 each for the theoretical and the practical exam. As far as I can tell, the difference is that in this case an instructor from the school comes along for the exam and they take care of registering you, possibly quicker than if you had to do it yourself (but there is only one evaluation lesson instead of two hours, go figure). Doing something like that with a local driving school could be the best solution for you but you will have to contact a few to see exactly what they are prepared to offer.
Now, here is what I could piece together about taking the exam independently from the (usually excellent and trustworthy) official website vosdroits.service-public.fr:
- You can technically register for the exam directly without going through a driving school (that's called being a “candidat libre”). But you need to have an experienced driver (“accompagnateur”) with you and a car with “double commands” (i.e. a brake and clutch pedal for the front passenger and a few other special features), which you might be able to rent from a school as mentioned above. The supervising driver is not allowed to receive any reward (presumably to prevent people from running unofficial driving schools).
- The website also mentions something about a mandatory 7-hour training for experienced drivers who want to supervise learners for the exam. Some unofficial websites imply that a court decision voided this requirement but the page linked above explicitly says that it's not true (or possibly not true anymore). As far as I understand, the whole thing is intended to allow charitable organizations to help people with low income get a license without paying a for-profit school (as it's difficult to get a job if you can't drive and difficult to pay for a license if you don't have a job). It's definitely not as simple as showing up with a friend's car and passing a short driving test.
- There are “special sessions” to the theoretical exam for people with special needs, including those who do not speak French well. Usually that means one session a month, in the chef-lieu of your department (whereas regular sessions are regularly organized in every place larger than a village, typically in town halls and the like). Some unofficial sources also mention something about coming with something like a “court translator” but I have not been able to ascertain this from any official source.
School or no school, an acquaintance who is involved in one of the charities I mentioned earlier just told me that there is a 7-month waiting time simply to get an exam spot in the département where I come from so brace yourself for some major inconvenience either way.
Also note that practicing the theory exam is probably useful, it has some quirks and is not merely about knowing a few well-known road signs or the like.