Learn Finnish: Hänellä on tapana ostaa kallis patukka viidellä eurolla.

This sentence uses the adessive case (-lla/-llä) to express the exact transaction price. When buying, selling, or renting something for a specific amount, Finnish uses this adessive of price. Notice that both the numeral (viidellä) and the currency (eurolla) must be in the adessive.

Why is the object kallis patukka in the nominative form instead of the genitive kalliin patukan?

This happens because of the hänellä on structure. When a sentence lacks a basic nominative subject (here, the person is in the adessive: hänellä), the total object of the verb takes the nominative form instead of the genitive. You see this same object rule with necessity verbs, like in Minun täytyy ostaa kallis patukka.

Why is the word for habit (tapa) in the essive case (tapana)?

The essive case ending -na/-nä expresses a state or a role, often translating to 'as a'. The literal translation of Hänellä on tapana is 'He has as a habit'. This is the standard idiom in Finnish for expressing that someone is in the habit of doing something.

Why does the number viisi change to viidellä in the adessive?