All my caminos except the last one via Francigena in Italy were walking.
I have the same issue coming from Singapore and wanted to do it on a bicycle but worried about the logistics. One of the earlier threads on this subject (couldn't find it now) was a couple that bought budget bikes for the Megastore Decathlon (France, UK, Spain, Italy...) there got rid of it at the end.
I followed what they did and got the same bike in Italy.
http://www.decathlon.es/C-1020933-mtb_todos-los-modelos
The model is basically the btwin rockrider 5.0. At 129.95 Euro currently, I added a 20 Euro rear rack and with my own panniers and own bottle cage I paid just under 150 euro. The bike is a tad heavy (hi-ten steel) but took in all the trails and bumps like a champ, mostly. When I arrived in Rome, I didn't have time to sell it but exchange it for one of the night's stay at the b&b, so on paper it cost less than a hundred euro. The owner told me later he didn't sell it but if any guest would like to use the bicycle to see Rome, it is there.
A few note on this bike.
1. Cheap components but has it great reviews and manages to stay above the Supermarket bicycle variety.
2. No suspension (which I prefer) and from the frame it looks more like a touring bike than a mountain bike.
3. The mtb size wheels was ok on some of the trails and gravel but didn't do too well on sand (maybe due to the semi-slick tires).
4. If you plan to ride the camino mostly on the road and some trails then it's perfect. But if you intend to follow the walker's route all the way, some of the rougher trails might be a challenge). I have a stretch so narrow and fallen trees and after 2 times throwing the panniers over and followed by the bike I gave up and took the road. I don't think you will have that on the
Camino Frances so it should be fine.
5. The riding position is slightly aggressive but if you keep to 70km or below a day it should be fine.
This is not for everyone on the bike. If you are used to good components and bikes back home, you may find it not up to your standard. For me I had the same bike in Singapore. It's my do everything bike so I am used to it, even the saddle. It's on option to consider as a 14 days rental in Spain will cost you almost double and a lot more in Italy.
Decathlon are in every major city in France, Spain and Italy. I figured for your case if you want to take up this option you would have to get it in Bayonne.
Thanks!!
