Question about доллар / доллара / долларов in survival kit lessons - Numbers, Decimals and Dollars

Здравствуйте!
There is something I don't understand about the word доллар in the lesson Numbers, Decimals and Dollars (survival kit lessons):

Это стоит двадцать долларов
Это стоит пятнадцать долларов
Это стоит пятьдесят девять долларов
доллары (dollars)
один доллар (one dollar)
два доллара (two dollars)

I guess it is related to grammatical cases, but why does the word доллар end in  -ов in some cases (when it's plural), ends in -ы for "dollars" and ends in -а for "two dollars"?

Спасибо

Hi Matthieu

These two web pages explain how noun endings change in Russian... 

http://learningrussian.net/russian-grammar/russian-nouns/agreement-with-numbers.php (noun endings for dollars).

http://masterrussian.com/aa071600a.shtml (noun ending changes in general).

Cheers

Louis

Thanks Louis!