Just returned from Spain a couple of weeks ago and was told about the vet in La Mailleraye-sur-Seine, France by another couple we met during our travels.
He charges from 15 Euros per dog, if you have brought your own dog worming tablets, which is what we were charged for our 30kg Labrador. I think it was approx. 25 Euros per dog (perhaps a little less) if you use the vets tablets. This is by far the cheapest we have been charged by any other vet we have used in France or Spain having paid anything between 35 to 45 Euros. We didn't make an appointment in advance, went about 3 o'clock and just had to return an hour later for the appointment. The receptionist doesn't speak English but showing her the pet passport and her pointing to the clock on the wall soon sorted things out! The vet is very pleasant and speaks good English.
There is a lovely (and very popular) aire down by the river Seine and only 5 minutes walk back up the road you have just driven down to the vets.
Another good vet we used in Germany about 3 years ago in Bernkastel-Kues only charged 16 Euros including the tablets!