Estaba sorprendido de lo caro que eran los zapatos.

[morbrorper]

English Translation

I was surprised at how expensive the shoes were.

Shouldn’t it be “lo caros”?

[keiths22]

I think that caro is being used as a noun in this sentence. The closest English sentence would be “I was surprised at the expensiveness of the shoes.”

[morbrorper]

Well, my grammar has this example: “Al menos tiene el coraje de señalar lo feas que son las camisas de los turistas.”

[Adrianxu]

both lo caro and lo caros work here