just saw this thread...it can be a GREAT experience, as long as you dont expect a party,its not like staying in a hostel..you might be the ONLY WOOFER ,places normally dont take many...another 1 or 2 people will be there if you are lucky...there arent that many commues/communities who take WOOFERS in spain(if you find one please let me know)
you honestly wont need much money at all...50pounds a week is more than enough! its good to have some cash so if you need a cheap accomodation til you sort out another farm if something doesnt work out..take a tent for that reason...if you find a place you like and have 2 days off a month no way will you need 50 pounds a week..unless you are an alkie and plan to drink LOADS in bars on yr day off
..another key point..some of the farms are quite far from the nearest bus stop,very very remote..so you might have to walk ages to GET to a town for a meal/drink/internet on yr day off.. yr hosts may or may not act as taxi service..
some people DO see WOOF as free labour and wont work along side you..they will sit in their million euro house and read astrology books

others see it as a cultural exchange and will work with you(this is great)and take you around places on the days off..sometimes the best places in spain are the ones you find via word of mouth, people not on WOOF lists.i dont want to be discouraging,cause it can be good esp if you dont mind NO internet and love nature..but ask them will they be working with you,and about transport(you want to be able to leave without walking 8miles to a bus stop,not unheard of in spain)