How much money is there? There are / is 50 dollars. (I guess, the correct option is "There is 50 dollars." because the verb "IS" is in the question, Is it correct?)
What are you doing? I'm writing a very important letter. OR I'm writing a letter very important.
There is 50 dollars.
I'm writing a very important letter.
Adjectives always come before the noun in English. You will never see or hear someone saying "a letter very important" in English.