How to prepopulate the currency of a commerce_price field
You can use Commerce Multi Currency module.


  This module enhances some of the multi-currency capabilities of Drupal
  Commerce.
  
  Current features:
  
  UI for fine granular definition of exchange rates. (Every currency
  combination can be defined separately if needed) Synchronization of
  currency exchange rates directly from the European Central Bank (ECB).
  UI to specify which rates shall be synchronized and which are handled
  manually. Generation of currency specific price fields inclusive
  generation of rule-set to handle them. hooks for easy integration of
  custom currency exchange rate sources.


This will give you a 'Price with currency' widget for your field.

And you will have a dropdown with all active currencies next to the price.