Learn German: Ein Euro für einen Schluck Wasser ist absurd. - One euro for a sip of water is absurd.

In German, when you talk about quantities, containers, or measurements followed by a noun, you simply put the two nouns next to each other. You do not need a preposition for 'of'. It is structured as 'a sip water' (ein Schluck Wasser) just like 'a glass of milk' becomes ein Glas Milch.

Why is it einen Schluck rather than ein Schluck?

It comes right after the preposition für (for). The preposition für always takes the accusative case. Since Schluck (sip) is a masculine noun (der Schluck), its article changes to the accusative form einen.

Why is Ein Euro in the nominative case if money is being exchanged?

Because the entire phrase Ein Euro für einen Schluck Wasser is the subject of the sentence, acting with the verb ist (is). The core structure is simply '[Subject] is absurd.' The money isn't the direct object of an action verb like 'to pay' here, so it stays in the nominative case.

If the sentence said 'two euros', would the word Euro take a plural ending?

Usually, no. It would just be zwei Euro. In German, masculine and neuter units of measurement or currency generally do not take a plural ending when they directly follow a number.