This is an interesting question. A traditional answer would be that una macchina is the direct object (complemento oggetto) and trentamila euro is a complemento di prezzo o stima, which, as you can see here, “quando dipende da verbi come pagare, costare, sborsare, ... si trova in forma diretta e si esprime senza preposizione” (when it depends on such verbs as pagare, costare, sborsare appears in a direct form and is expressed with no preposition). So, trentamila euro is direct, but it is not the object of the verb.
On the other hand, the very notion of complementi is quite controversial. There are more modern approaches to grammar (of which I am no expert) in which each verb admits zero or more arguments; in this viewpoint, the verb pagare admits 1 to 3 arguments (one argument is the subject, and it may be the only one, as in “Andiamo, [io] ho pagato”; or a second one may appear: “[Io] Pago un caffè”; or a third too: “Gianni ha pagato questa macchina trentamila euro”). If your Italian is up to it, you can read about some alternative approaches in the article about “Complementi” in the Treccani Enciclopedia dell'italiano, as well as in the article about “Argomenti”.