Conjugating the adjective results in a slightly different meaning. The adjective and verb don’t have to be in the same tense. So, for example:

高い本を買いました = I bought the book which is expensive (as in, it’s still expensive)

高かった本を買いました = I bought the book that was expensive (but the price has fallen since then)

Also,

高い本を買います = I’m buying the expensive book

高かった本を買います = I’m buying the book that used to be expensive

To Joel’s point, I think, if you were trying to emphasize the fact that the adjective no longer applies the the noun, you would want to add modify the statement more than just changing the verb conjugation. For example:

Although “高かった本を買います” does mean “I’ll by the book that was expensive,” I think if you were trying to emphasize that it no longer is, you would want to add additional information. A more natural way to do so in English would be ” I’ll by that book that used to be expensive.” Using “used to be” has a stronger implication of the fact that it no longer is than “was”. Saying something like “高くなくなった本を買います” (lit. “I’ll by the book that came to be inexpensive”)  might be a better way to bring emphasis.

You might also try “以前高かった本” but I’m not actually sure about the grammar here. Maybe someone else can chime in.

Yeah, this went a little beyond what you were asking.

By the way なる is a verb that roughly means “to become”.
高く… is the adverbial (precedes verbs) form of 高い.　( い –> く )
Putting the two together gives you 高くなる, “to become expensive.”
The adverbial form of the negative 高くない, is 高くなく. ( い –> く )
Putting that with なる gives you 高くなくなる, “to become inexpensive.”
Converting the なる part to the plain past tense gives you 高くなくなった, “became inexpensive.”

In Japanese, anything that describes a noun (even if it’s a long phrase) comes before the noun.
So sentences like “The man I saw yesterday” get rearranged to “The I-saw-yesterday man.”
“The book that came to be inexpensive” becomes “The became-inexpensive book” or 高くなくなった本.