Welcome to the forum and as you wanted some help & ideas on costs hope this will help.
First of all 95% of commercial lakes dont require a licence.
Some lakes have bait restrictions but I would reckon 5 - 10 kgs boilies, 10 kgs pellets & the same particles, this will obviously depend on the lake concerned.
Costs to take into consideration once your in France:-
The further into France you drive the more it costs toll wise and I would estimate 10 euros per 100 miles on top of your petrol. Food prices in the supermarkets are more expensive although beer is slightly cheaper. If you can get a good food package for between 100 & 110 euros a week it is well worth having & saves trips to the supermarket.
If you havent picked a lake yet I would make a shortlist of say 6 look on the net for reports of lakes, email each of them and ask them as many questions as you can, its an important decision. I am sure they would all respond quickly.
Hope this will help
Good luck!