Learn Dutch: Ik betaal vijftig euro.

In Dutch, words for currencies, weights, and measurements usually stay in the singular form when they come after a specific number. So it is always vijftig euro, never vijftig euro's.

How do you pronounce the -ig at the end of vijftig?

The ending -ig in Dutch numbers is pronounced softly, like "uh-ch" (using the typical Dutch "ch" or "g" sound in the back of the throat). You do not pronounce the "i" like "ee" or the "g" like in the English word "pig".

The dictionary form of the verb is betalen (to pay). Why does the "I" form betaal have a double "a"?

This is because of the Dutch rule for keeping vowels long. In be-ta-len, the a is long because it ends a syllable. When you drop the -en to make the "I" form, the syllable is closed by the l. To keep the "a" sounding long, you must double it to write betaal.