Don't believe that article. I can't find this season's prices, but last season (2014/15) the prices were between 133€ and 1097€ for a full season ticket (25% discount for league-only):
http://www.fcbarcelona.es/socios/detalle/ficha/pago-abono-estadio-palau-y-parquing-temporada-2014-15
Plus a season ticket holder has to be a member, and that's a separate 170€ (approx) charge per year. So a minimum of about 270€ for a league-only ticket, not the £74 claimed by the Independent.
Also, don't also believe attendance figures - e.g. Arsenal equate attendance with "tickets sold" (including season tickets), and not the actual attendance:
http://aisforarsenal.blogspot.co.uk/2014/08/arsenals-actual-average-attendance-last.html
I don't think Barcelona's attendance "problem" (I'm sure 99.9% of other clubs would kill for an average of 75K+) is only down to season ticket holders. General ticket prices have always seemed exhorbitantly expensive to me, mostly bought by tourists. But then I always try to buy a decent seat for decent opponents.
And I don't think the renovation of the Camp Nou is in order to increase the seating capacity - that's just a positive side-effect of the design (levelling out the top tiers all round the stadium and installing a roof). The main reason is that it's old, cramped, lacking in internal space for facilities (thereby reducing matchday income) and in need of repair.