hi Miguel, try...
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1)
www.bravobike.comrent a bike page at
www.bravobike.com/rent_bike_eng.asp10-15�/day depending on the duration of the rental, seems reasonable to me.
they even have an organized camino de santiago bike tour at
www.bravobike.com/tours_camino_santiago_eng.asp (a bit steep on the price though

).
2)
www.madridbiketours.comno rental page i can see but i am sure they offer the rental service only, you may want to contact them.
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if they are renting for 15 or more days, they may want to buy bikes and then try to sell them when they leave (i might be interested in buying them myself). a 15 day rental would run 150 Euros and for a bit more they could buy bikes.
i got some nice and pretty inexpensive bikes for ena and myself at
BICICLETAS OTERO here in madrid on calle segovia.
i know you are busy with your studies but please come back and let us knwo how it goes if your friends end up renting bikes in spain.
saludos,
jer...