You can set up a Number question so that the answer is automatically formatted as an amount of money.
To set this up, first configure your Number question by hovering over it in the editor and clicking the cogwheel icon to the right. Then, scroll down to the "Number input formatting" setting and change it to "Currency."
The answer will be formatted according to the currency that you have set for the form. You can switch your form to a different currency (USD, GBP, etc.) under Configure → Payments → Currency.
If you like, you can also change the default currency for new forms in your Account Settings. Please note however that an individual form's currency setting will always take precedence over the account's currency setting.