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Full list of accomodations in Jaca
I can't recommend one though.
Guide: H=Hotel Hs=Hostal P=Pensión Ap=Apartment

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P.S.: It could be easier to take a look at webs like hostelworld, hostelbookers or, maybe, even booking (although in this case you should look for the cheapest accomodation available as it lists all sort of accomodations). All of them will show you accomodations in Jaca although not all the accomodations in Jaca. The accomodations showed in Hostelworld (and in hostelbookers although sometimes it includes not so budget accomodations) should be budget. In booking you'll find all sort of accomodations, look for the cheapest one to see if it's within your budget.

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Ok thanks for the link, appreciate that! Looks like there is a hostel there after all, dorms for 15e a night, so that could be a goer. Will try calling some of those other pensions too. Cheers!