17.5 refers to a the price of a kilogram of cocaine which is around $17,500. Mr. refers to lyrics from the rapper Young Jeezy on his albumn Thug Motivation: The Inspiration 102 as he being Mr. 17.5.
"Snow man, get cha' hands up high
It's ya' boy, mr 17.5" - Young Jeezy "Mr. 17.5"