This is a link with Dunkerque Marina (along the Channel/20 NM from UK) North of France)... I suppose it is one of the cheapest marina in France
... With all services and a yard in order to winter on land if you prefer it. (there are 3 different marinas
Just click on "tarifs" and you will have the price
list (1600 €/year afloat for <10,50 m length)...
Why is it cheap
.? Mostly because Dunkerque is mostly (or was) an industrial city and a non touristic one.
But, it can be a very convenient spot for wintering.
Ports de plaisance Dunkerque Neptune