The value of the property & the official rated value , valor catastral, are not linked. The valor catastral is normally based on the original cost of construction , normally about 33% of the selling price on a recent ly built property & in the case of very old properties, like mine , an assumed estimate of what the cost of construction would have beeen when it was built.
The official rated value = valor catastral
probably there might also be a 'valor suelo' = any additional land. the 'valor construcción' is added to this resulting in the 'Base liquido' = what you are paying your tax on .
'tipo' or 'tipo gravamen' is the percentage of 1 % which you have to pay.
I:e. if the v. const. is 9900€ +100€ v. suelo = 10,000€ x 0,75%( Tipo) = 75€ IBI.
It's got nothing to do with what the house buys & sells for.