The cheapest BPA free bottles I've found are made by MAM. They have a special anti-colic system as well. I think a 9 ounce bottle costs around 8 or 9 francs at the pharmacy.
You could also check
allforkids I'm pretty sure I saw BPA free bottles there at reasonable prices.
Having said all that, Coop and Migros also carry a small selection of BPA free prodcuts