Handling currencies in a database
There is no answer to that - it depends. If you are a smaller outfit that with locality (like a dozen shops in the USA) then all you really care about is the price in USD. If you are an international retailer with websites in many countries with local pricing, you need to keep likely even multiple prices per product in different currencies.


  Let's say I have Product table, this product has a price, but what currency 
  should the price be in?


It should not be there. it assumes not only one currency (which wold be the base currency the system runs in) but also no rebate system at all. Pricing, except for really simplistic systems, is a separate table.

Suggested reading: Data Model REssouce Book, Volume 1 - whole chapter on storing prices.

If yo ustore it like this, it is either base currency (stored somewhere central) or yes, you add a currency id field to reference the currency. I suggest using the ISO currency code for that, and adding negative numbers if needed when no official code is assigned.