Welcome to AAM, vblancas.
The declared value will be in the
Escritura (Deed of title), and you shouldn't buy a property until you have obtained a current extract of it (
Nota Simple) from the Land Registry. The declared value may be significantly less than the actual price paid for the property, in time-honoured tax-dodging tradition...
The rateable value or
valor catastral is the official assessed value of the property, on which your property owner's Income Tax will be based. I don't know quite how it's calculated, but a lawyer who deals with property transactions should be able to advise. It will be listed on the the receipt(s) for the paid Rates (
Impuestos sobre Bienes Inmuebles, or I.B.I.), which is the annual real estate tax; you should also check with the Town Hall that the I.B.I. has been fully paid up to date, otherwise you may 'inherit' an additional liability.
www.spanishpropertyinsight.com (to which I have no connection) has some useful resources.