President Nixon Family in the White House

When my brother was a kid, he wrote a letter to President Nixon and received a autograph picture of the President and his family in the White House.

The picture was hand signed by the president, I think in blue ink like a sharpie or marker and not a pen. I believe it is personalized and written specifically for my brother in response to his letter.

I'm not sure what it says because it's put away in a box somewhere. I heard years ago it was worth more than some because it was hand signed and addressed to my brother by name and was signed while he was President.

I thought it would be a nice surprise for my brother if it turned out to be valuable to collectors. Can anyone give an idea to the value?