There are a number of mills around - one of the best (that I have found) is Moli D'Oli just off the road from Tortosa to L'Aldea - if you have a look through the posts on the forum you'll find them mentioned elsewhere.

As for price, well it's very variable so impossible to give you a precise figure - I'm sure someone out there has taken some in recently and can give you some idea of a recent price but it can change daily.

Nigel