How did Microsoft determine the amount of currency added to my Microsoft account?
To make this a seamless and positive experience for our customers, we have considered many factors to help ensure that the amount added to your Microsoft account is of equal or greater Xbox stores' value than your Microsoft Points. The value of Microsoft Points in Xbox stores may not always be equivalent to retail price. Retail price is determined by the retailer, whereas the Xbox stores' value reflects the value of Microsoft Points throughout Xbox stores.
2100 Microsoft Points Card (Purchased in Spain)
18€ Paypal
How did Microsoft determine the amount of currency added to my Microsoft account?
To make this a seamless and positive experience for our customers, we have considered many factors to help ensure that the amount added to your Microsoft account is of equal or greater Xbox stores' value than your Microsoft Points. The value of Microsoft Points in Xbox stores may not always be equivalent to retail price. Retail price is determined by the retailer, whereas the Xbox stores' value reflects the value of Microsoft Points throughout Xbox stores.
Source -> http://support.xbox.com/en-US/billing/microsoft-account-balance/microsoft-points-retire-faq