How much is 500 pesetas in US Dollars?

Spain has not used the peseta as its currency since 2002, when the euro was adopted. At that time the conversion rate was

1 USD = 122.6 Spanish Pesetas

=> 500 pesetas = 4.1 USD

If you still have peseta coins or bills it may be difficult to exchange them for dollars or euros without going through a major bank. The conversion costs for a small amount of an obsolete currency would probably exceed its actual value.